Make all the locks exception-safe.

Reduce header clutter.

Similar to boost::scope_exit, but fewer include dependencies.

Fixes build failure when Vorbis is disabled, but FLAC is enabled
(HAVE_XIPH is true, but HAVE_OGG is false).

For easier state management inside filter plugins.

Comes with a callback pointer instead of a virtual method, which
allows multiple instances in one class.

Replaces the old BoundMethod template.

Prepare to reuse the same code for a new Vorbis decoder plugin based
on libvorbis instead of libvorbisfile.

Some "ar" implementations don't allow empty archives, and failed to
build it.  The real reason (which is hidden with GNU binutils) is that
our Makefile.am was incorrect, and didn't completely disable
libxiph.a.

See https://bugs.musicpd.org/view.php?id=4526