release v0.20.12

Closes #155

Fixes a deadlock caused by commit
31ab78ae.  That commit was not
actually bad - just these two calls have always been bad, which went
unnoticed for a long time.

Avoid another potential deadlock: if no file descriptors are
registered, our non-blocking ALSA code cannot ever work.

After UnlockActivate() returns, we not only need to check for errors,
but also for more room in the ring buffer.  If we don't check the ring
buffer, it may be drained already, and the cond.wait() call will never
finish.

Closes #151

Bugs found by Coverity.

Without the flush, ReadPage() may not return any data, or not all
data.  This may result in incomplete ddata the new "header" page,
corrupting streams with some encoders such as Vorbis.

Fixes #145

Don't require a quality or bitrate setting.  If nothing is set, don't
fail startup - just go with a good default.  A quality setting of 3 is
what "oggenc" defaults to as well.

InputStream::Read() and InputStream::Seek() are called with the mutex
locked.  That means the implementation must not block, or unlock the
mutex before calling into blocking code.

Previously, a slow CD drive could stall the whole MPD process,
including the main thread, due to this problem.

Closes #149

Work around -Werror=catch-value.

Workaround for #146.

Move its code to DeferEvent instead.

Allows defining a list of supported audio formats, and allows
switching on and off DoP with certain formats.

This is a first rough draft.  The setting syntax and its semantics may
still be redesigned.