Fixes missing SongBorder() call, which causes "single" mode breakage.

If there's a stream tag, don't let the song tag override it in the
next update_stream_tag() call.

If the song tag comes from a stream, and MPD playback restarts, MPD
would believe the tag should override the newly received tag.  This
makes the previous tag appear stuck.  This change passes the song tag
only if it's authoritative - i.e. if it's a song file.

Reduce runtime overhead.

Right after booting, the monotonic clock starts with a very small
value, and AudioOutput::LockUpdate() may believe that the fail_timer
has not recovered yet.

When seeking to the beginning of a packet, skip the samples that come
before the desired time stamp.

Ask FFmpeg to seek to the next packet boundary *before* the seek
position, so we don't miss audio data.  Now we get too much, but we'll
solve that in the next commit.

Fixes deadlock on small responses.

zlib cannot be built out-of-tree, and previously, we were building in
the "src" directory, not in the "build" directory.  The
make_build_path() return value was unused.

The Connect method can be called between Schedule and lock. In that case, when
locked, the state is already set to CONNECTING of READY and the condition won't
be signaled anymore.

Not initialising granulepos leads to it having arbitrary values in the
encoded stream including possibly negative values which are not valid
and confuse opusdec.  Explicitly initialise opus_encoder::granulepos
to avoid that problem.