Preparing for per-plugin configuration sections in mpd.conf.

Some plugins used the APE or ID3 tag loader as a fallback when their
own methods of loading tags did not work.  Move this code out of all
decoder plugins, into song_file_update().

Free the tag object when it turns out to be empty.  This simplifies
several functions and APIs.

Use tag_is_empty() instead.

Renamed types, functions, variables.

Use GLib's G_GNUC_UNUSED instead of gcc.h's mpd_unused.

SEEK_SET is defined by unistd.h.  Explicitly include it.

Refuse to play audio formats which are not supported by MPD.

Define the special value "-1" as "unknown size".  Previously, there
was no indicator for streams with unknown size, which might confuse
some decoders.

The stream_decode() and file_decode() methods returned a boolean,
indicating whether they were able to decode the song.  This is
redundant, since we already know that: if decoder_initialized() has
been called (and dc.state==DECODE), the plugin succeeded.  Change both
methods to return void.

Renamed functions and variables.

Instead of having a seprate try_decode() method, let the
stream_decode() and file_decode() methods decide whether they are able
to decode the song.

The flac plugin wasn't initialized properly when an OGG file was being
decoded.  For some reason, flac_process_metadata() was explicitly not
called for OGG files.  Since that seems to fix the issue, make it
always call flac_process_metadata().

Since decoder_list.c does not include the libflac headers, it cannot
know whether to add the oggflac plugin to the decoder list.  Solve
this by always enabling the oggflac sub-plugin, even with older
libflac versions.  When the libflac API cannot support oggflac,
disable the plugin at runtime by returning "false" from its init()
method.

Disable flac's "oggflac" sub-plugin when libflac does not support
ogg-flac files.

Instead of checking the stream_types bit set, we can simply check
whether the methods stream_decode() and file_decode() are implemented.

Don't pass the "seekable" flag with every decoder_data() invocation.
Since that flag won't change within the file, it is enough to pass it
to decoder_initialized() once per file.

The strings were constant, but the pointers weren't.  C syntax is
somewhat tricky..

All decoder_plugin structs are initialized at compile time, and must
never change.

Don't return 0/-1 on success/error, but true/false.  Instead of int,
use bool for storing flags.

Call ob_clear() in decoder_command_finished() instead of implementing
that call in every decoder plugin.

For boolean values and success flags, use bool instead of integer (1/0
for true/false, 0/-1 for success/failure).

Renamed all functions and variables.

Everybody should use struct input_stream.

These plugins are not input plugins, they are decoder plugins.  No
CamelCase in the directory name.

Don't compile the sources of disabled decoder plugins at all, and
don't attempt to register these.

The last bit of CamelCase in audio_format.h.  Additionally, rename a
bunch of local variables.

"bool" should be used in C99 programs for boolean values.

When there are standardized headers, use these instead of the bloated
os_compat.h.

The old struct initializers are error prone and don't allow moving
elements around.  Since we are going to overhaul some of the APIs
soon, it's easier to have all implementations use C99 initializers.

Why have a "_func" prefix on all method names?  Also don't typedef the
methods, there is no advantage in that.

There is still a lot of duplicated code in flac_plugin.c and
oggflac_plugin.c.  Move code from flac_plugin.c to _flac_common.c, and
use the new function flac_common_write() also in oggflac_plugin.c,
porting lots of optimizations over to it.

The previous patch on this topic was incomplete: it still added
data->chunk_length when calling flac_convert().  Remove this, too.

The inline function audio_format_sample_size() calculates how many
bytes each sample consumes.  This function already takes into account
that 24 bit samples are 4 bytes long, not 3.

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

It was possible for the decoder thread to go into an endless loop
(flac and oggflac decoders): when a "STOP" command arrived, the Read()
callback would return 0, but the EOF() callback returned false.  Fix:
when decoder_get_command()!=NONE, return EOF==true.