Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

Decouple some more from GLib.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Drop support for the old faacDec* API.

Hide the definition from C code, to prepare the transition to C++.

Pass a callback table to scan_file() and scan_stream(), instead of
returning a tag object.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

Make it X_decoder_plugin.c.

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

This patch changes the following decoder plugins to implement
stream_tag() instead of tag_dup():

 faad, ffmpeg, mad, modplug, mp4ff, mpcdec, oggflac

This simplifies their code, because they do not need to take care of
opening/closing the stream.

Remove the data_time parameter from decoder_data().  This patch
eliminates the timestamp counting in most decoder plugins, because the
MPD core will do it automatically by default.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.

Let the audio_check library verify the audio format in all (relevant,
i.e. non-hardcoded) plugins.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

When the ID3 tag in an AAC file is larger than the current buffer, the
function decoder_buffer_consume() aborts.  By using the new function
decoder_buffer_skip() instead, we can safely skip the ID3 tag.

It makes no difference right now, but we're about to add an endianness
flag and will want to make sure it's correctly initialised every time.

On failure, the function should return NULL, not a boolean.