Pass the client struct instead of the raw file descriptor.

String literals (including those defined in CPP macros) can be
concatenated at compile time.  This saves some CPU cycles in
vsnprintf() at run time.

commandError() has been superseded by command_error(), and is not
being used anymore.  Remove it.

The function loadPlaylist() wants to report incremental errors to the
client, for this reason we cannot remove its protocol dependency right
now.  Instead, make it use the client struct instead of the raw file
descriptor.

Don't pass the raw file descriptor around.  This migration patch is
rather large, because all of the sources have inter dependencies - we
have to change all of them at the same time.

Pass the client struct to CommandHandlerFunction and
CommandListHandlerFunction.  Most commands cannot take real advantage
of that yet, since most of them still work with the raw file
descriptor.

Instead of passing the file descriptor, pass the client struct to
getCommandEntryAndCheckArgcAndPermission().

These two functions take a client struct instead of the file
descriptor.  We will now begin passing the client struct around
instead of a raw file descriptor (which needed a linear lookup in the
client list to be useful).

No protocol code in the audio output library.

The "volume" library shouldn't talk to the client.  Move error
handling to command.c.

Again, move error handling to command.c.

Move another ocurrence of error handling over to command.c.

This patch continues the work of the previous patch: don't pass a file
descriptor at all to traverseAllIn().  Since this fd was only used to
report "directory not found" errors, we can easily move that check to
the caller.  This is a great relief, since it removes the dependency
on a client connection from a lot of enumeration functions.

Database traversal should be generic, and not bound to a client
connection.  This is the first step: no file descriptor for the
callback functions forEachSong() and forEachDir().  If a callback
needs the file descriptor, it has to be passed in the void*data
pointer somehow; some callbacks might need a new struct for passing
more than one parameter.  This might look a bit cumbersome right now,
but our goal is to have a clean API.

Continuing the effort of removing protocol specific calls from the
core libraries: let the command.c code call commandError() based on
PlaylistInfo's return value.

Return an "enum playlist_result" value instead of calling
commandError() in storedPlaylist.c.

The playlist library shouldn't talk to the client if possible.
Introduce the "enum playlist_result" type which the caller
(i.e. command.c) may use to generate an error message.

Make them both return void.

Client's input values should be validated by the command
implementation, and the core libraries shouldn't talk to the client
directly if possible.  Thus, setPlaylistRepeatStatus() and
setPlaylistRandomStatus() don't get the file descriptor, and cannot
fail (return void).

When an error occurs after the file has been opened, the function will
never close the FILE object.

The "fspath" argument of writeStoredPlaylistToPath() must never be
NULL.  There should be an assertion on that, instead of a run-time
check.

The function valid_playlist_name() checks the name, but it insists on
reporting an eventual error to the client.  The new function
is_valid_playlist_name() is more generic: it just returns a boolean,
and does not care what the caller will use it for.  The old function
valid_playlist_name() will be removed later.

Currently, when the tag cache is being serialized to hard disk, the
stdio buffer is flushed before every song, because tag_print.c
performs unbuffered writes on the raw file descriptor.  Unfortunately,
the fdprintf() API allows buffered I/O only for a client connection by
looking up the client pointer owning the file descriptor - for stdio,
this is not possible.  To re-enable proper stdio buffering, we have to
duplicate the tag_print.c code without fprintf() instead of our custom
fdprintf() hack.  Add this duplicated code to tag_save.c.

Move everything which dumps song information (via tag_print.c) to a
separate source file.  song_print.c gets code which writes song data
to the client; song_save.c is responsible for serializing songs from
the tag cache.

Move everything which dumps a tag to a file descriptor to tag_print.c.
This relaxes dependencies and splits the code into smaller parts.

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.