Replacing GLib macros.

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

Replaces GLib's GError.

DSD-over-USB should not be a MPD core format, because it is not a
"natural" format; it is just a temnporary over-the-wire format.  This
format has been implemented in pcm_export, and does not need to be
supported by pcm_convert.

For 32 bit DSD-over-USB support.

Prepare for removing SAMPLE_FORMAT_DSD_OVER_USB.

For simplicity, the MPD core should not have to deal with packing.  It
is rarely used, and those plugins that need it should use the
pcm_export library instead.

Converts padded 24 bit samples to packed 24 bit samples.  Will replace
the packed S24 sample format, which is not used internally.

Improving oss_reopen() by using the very same value that was used
initially.

Don't use audio_format.reverse_endian.

This format is unused since the DSDIFF decoder plugin now reverses the
bit order.

Make sure we get a UTF-8 encoded string.

Basic support for Direct Stream Digital.  No conversion yet, and no
decoder/output plugin support.

Support for conversion from float to 16, 24 and 32 bit integer
samples.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Move the "extern" declarations from output_list.c, for more type
safety.

See code comment.

According to the Solaris dsp manpage, AFMT_S24_PACKED is
little-endian:

 http://download.oracle.com/docs/cd/E19963-01/821-1475/6nmf5baot/index.html

The Minix soundcard.h header says the same.

Remove trailing whitespace found by this command:
find -name '*.[ch]' | xargs grep "[[:space:]]$"

MPD doesn't have child processes anymore, and thus we're not expecting
to receive SIGCHLD very often.  Since hard disk access isn't
interrupted by signals anyway, we don't need those excessive checks.

Added #ifdefs on the macros AFMT_S24_NE and other OSS4 extensions.