Resampling 32 bit samples is the same as resampling 24 bit samples -
both are stored in the int32_t type.

Some 24 bit code can be reused.  The 32 bit variant has to use 64 bit
integers, because 32 bit integers could overflow.  This may be a
performance hit on 32 bit CPUs.

This is the first patch in a series to enable 32 bit audio samples in
MPD.  32 bit samples are more tricky than 24 bit samples, because the
integer may overflow when you operate on a sample.

audio_valid_sample_format() verifies the number of channels.  Let's
just say up to 8 channels is allowed (which is possible with some
consumer sound chips).  I don't know if there are bigger cards, and
since I cannot test it, I'll limit it to 8 for now.

Do error reporting with GLib's GError library in this library, too.

Do error reporting with GLib's GError library.

Renamed a bunch of variables.

Don't abort MPD when a sample format is not supported by pcm_volume().

Parse the vorbis comments in libflac's metadata_callback and pass them
as tag struct to the decoder API.

When the unix domain socket path is too long, don't abort with
g_error().

Use config_get_block_string() and manual GError handling instead.

Do error handling with GError instead of aborting with g_error().

Don't hard code the "bits" parameter to 16.  Try to use the input's
sample format, if possible.

The MPD core logs the audio format of all audio outputs.  Remove the
duplicate message from the plugins.

To aid debugging, print the audio format of the output plugin in a
debug message, and print information about PCM conversion.

To aid debugging, print the audio format of the decoder plugin in a
debug message, and print information about PCM conversion.

During the listen_add_host() API transition, the windows code wasn't
tested, and several removed arguments are still in use there.

Windows doesn't have the standard headers sys/socket.h and netdb.h.

Even when --enable-documentation is passed, don't abort if "xmlto" was
not found.

The output path must be ./doc/protocol/, not ./protocol/

Renamed functions and variables.

Use GLIB_CHECK_VERSION() instead of manually checking
GLIB_MAJOR_VERSION, ...

On some hardware, reading the mixer value from hardware is an
expensive operation, and MPD has to do it for every client.  Throttle
access to the hardware, cache the result for one second.

time() is not a monotonic timer, and MPD might get confused by clock
skews.  clock_gettime() provides a monotonic clock, but is not
portable to non-POSIX systems (i.e. Windows).  This patch uses GLib's
GTimer API, which aims to be portable.

The option "enabled" is on by default.  If you specify "enabled no" in
an audio_output section, then this device is disabled by default.

decoder_data() returns a decoder_command, no need to call
decoder_get_command() twice after decoder_command().

If an input_stream is not seekable, libaudiofile fails to play at all:

 Audio File Library: unrecognized audio file format [error 0]

Since we know in advance whether the input_stream is seekable, just
refuse to play on a non-seekable stream.

Renamed several variables and a function.