Based on client_puts(), client_printf() is the successor of
fdprintf().  As soon as all fdprintf() callers have been rewritten to
use client_printf(), we can remove fdprintf().

client_write() writes a buffer to the client and buffers it if
required.  client_puts() does the same for a C string.  The next patch
will add more tools which will replace fdprintf() later.

As usual, include only headers which are really needed.

clearMpdTag could be called on a tag that was still in a
tag_begin_add transaction before tag_end_add is called.  This
was causing free() to attempt to operate on bulk.items; which is
un-free()-able.  Now instead we unmark the bulk.busy to avoid
committing the tags to the heap only to be immediately freed.

Additionally, we need to remember to call tag_end_add() when
a song is updated before we NULL song->tag to avoid tripping
an assertion the next time tag_begin_add() is called.

client->fd becomes -1 when the client expires.  Don't use FD_ISSET()
with this expired client; doing so would cause a crash due to SIGBUS.

Since client->fd==-1 has become our "expired" flag, it may already be
-1 when client_close() is called.  Don't assert that it is still
non-negative, and call client_set_expired() instead.

The usual bunch of const pointer conversions.

Don't call tag_pool_get_item() for duplicating tags, just increase the
item's reference counter instead.

Fix lots of "unused parameter" warnings in the OggFLAC decoder
plugin.  Not sure if anybody uses it anymore, since newer libflac
obsoletes it.

During the tag library refactoring, the shout plugin was disabled, and
I forgot about adapting it to the new API.  Apply the same fixes to
the oggflac decoder plugin.

Yet another patch which converts pointer arguments to "const".

While parsing the tag cache, don't allocate the directory name from
the heap, but copy it into a buffer on the stack.  This reduces heap
fragmentation by 1%.

If many tag_items are added at once while the tag cache is being
loaded, manage these items in a static fixed list, instead of
reallocating the list with every newly created item.  This reduces
heap fragmentation.

Massif results again:

 mk before:  total 12,837,632; useful 10,626,383; extra 2,211,249
 mk now:     total 12,736,720; useful 10,626,383; extra 2,110,337

The "useful" value is the same since this patch only changes the way
we allocate the same amount of memory, but heap fragmentation was
reduced by 5%.

The function newNullSong() is only used internally in song.c.

Try to detect if the string needs Latin1-UTF8 conversion, or
whitespace cleanup.  If not, we don't need to allocate temporary
memory, leading to decreased heap fragmentation.

Same as the previous patch, prepare the function fix_utf8() this time.

At several places, we create temporary copies of non-null-terminated
strings, just to use them in functions like validUtf8String().  We can
save this temporary allocation and avoid heap fragmentation if we
add a length parameter instead of expecting a null-terminated string.

We must never pass value==NULL to fix_utf().  Replace the run-time
check with an assertion.

Since the inline function cannot modify its caller's variables (which
is a good thing for code readability), the new string pointer is the
return value.  The resulting binary should be the same as with the
macro.

The new source tag_pool.c manages a pool of reference counted tag_item
objects.  This is used to merge tag items of the same type and value,
saving lots of memory.  Formerly, only the value itself was pooled,
wasting memory for all the pointers and tag_item structs.

The following results were measured with massif.  Started MPD on
amd64, typed "mpc", no song being played.  My music database contains
35k tagged songs.  The results are what massif reports as "peak".

 0.13.2:     total 14,131,392; useful 11,408,972; extra 2,722,420
 eric:       total 18,370,696; useful 15,648,182; extra 2,722,514
 mk f34f694e: total 15,833,952; useful 13,111,470; extra 2,722,482
 mk now:     total 12,837,632; useful 10,626,383; extra 2,211,249

This patch set saves 20% memory, and does a good job in reducing heap
fragmentation.

The value is stored in the same memory allocation as the tag_item
struct; this saves memory because we do not store the value pointer
anymore.  Also remove the getTagItemString()/removeTagItemString()
dummies.

This patch makes MPD consume much more memory because string pooling
is disabled, but it prepares the next bunch of patches.  Replace the
code in tagTracker.c with naive algorithms without the tree code.  For
now, this should do; later we should find better algorithms,
especially for getNumberOfTagItems(), which has become wasteful with
temporary memory.

This prepares the following patches, which aim to reduce MPD's memory
usage: we plan to share tag_item instances, instead of just their
values.

The ID3 code uses only the public tag API, but is otherwise
unrelated.  Move it to a separate source file.

Since tag_new() uses xmalloc(), it cannot fail - if we're really out
of memory, the process will abort.

Getting rid of CamelCase; not having typedefs also allows us to
forward-declare the structures.

Unfortunately, the C standard postulates that the argument to free()
must be non-const.  This does not makes sense, and virtually prevents
every pointer which must be freed at some time to be non-const.  Use
the deconst hack (sorry for that) to allow us to free constant
pointers.

Instead of passing the pointer to the "expired" flag to
processListOfCommands(), this function should use the client API to
check this flag.  We can now remove the "global_expired" hack
introduced recently.

Start exporting the client struct as an opaque struct.  For now, pass
it only to processCommand() and processListOfCommands(), and provide a
function to extract the socket handle.  Later, we will propagate the
pointer to all command implementations, and of course to
client_print() etc.

Change the order of function declarations in client.h, to make it well
arranged and readable.

The old code tried to write a response to the client, without even
checking if it was already closed.  Now that we have added more
assertions, these may fail...  perform the "expired" check earlier.

Patch bdeb8e14 ("client: moved "expired" accesses into inline
function") was created under the wrong assumption that
processListOfCommands() could modify the expired flag, which is not
the case.  Although "expired" is a non-const pointer,
processListOfCommands() just reads it, using it as the break condition
in a "while" loop.  I will address this issue with a better overall
solution, but for now provide a pointer to a global "expired" flag.

And again, convert arguments to const.

client_defer_output() was modified so that it can create the
deferred_send list.  With this patch, the assertion on
"deferred_send!=NULL" has become invalid.  Remove it.

I'm really no fan of the warning log, it's too complex
for how little it gets used; but fixing it is another
problem.

Warren hasn't been active in development in a while and probably
doesn't have much time to answer user questions.  So point the
mailing contact to the public mailing lists where any developer
can see and answer.

Why waste 4 bytes for a flag which we can hide in another variable.