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.

Hiding this flag allows us later to remove it easily.

Unclutter the client_new() constructor by moving unrelated complex
code into a separate function.

The last patch removed the "continue" directive, and now the while
loop is without function.  Remove it.  Also make client_manager_io()
return 0.

Previously, when select() failed, we assumed that there was an invalid
file descriptor in one of the client structs.  Thus we tried select()
one by one.  This is bogus, because we should never have invalid file
descriptors.  Remove it, and make select() errors fatal.

Eliminate duplicated code, call client_defer_output() which we
splitted from client_write_output() earlier.

Move the second part of client_write_output() into a separate
function.

client_defer_output() was designed to add new buffers to an existing
deferred_send buffer.  Tweak it and allow it to create a new buffer
list.

Exit the function when an error occurs, and move the rest of the
following code one indent level left.

Split the large function client_write_output() into two parts; this is
the first code moving patch.

All of the client's resources are freed in client_close().  It is
enough to set the "expired" flag, no need to duplicate lots of
destruction code again and again.