Merge lots of duplicate code.

Decouple some more from GLib.

Replacing GLib macros.

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

Replaces GLib's GError.

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

functions to new dsdlib. Update user doc.

Version 4 of my patch to add DSF support to the DSDIFF
decoder plugin.

This time I have taken a different approach and created a new
read_metadata function specific for reading DSF files. This saves an
indent (and for me a lot of indent nightmares) and also useful for
splitting the DSF and DFF decoders later on.

There are still a few lines which exceed the 80 character width limit by
a few chars. I was not able to stay within the limit and create (for me)
readable code.

Jurgen

Allow to remove this complexity from the MPD core.

Move the responsibility for the conversion to the PCM library.  This
will allow passing the verbatim DSD samples to an output plugin.

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

Eliminate the conversion to integer samples, the MPD core can do this
now.

Defaults to "no", which fixes the noise problems.

Doesn't seem to work yet, getting just noise from a test file.
Seeking isn't implemented yet.