When switching to another song manually, the player checks if the
decoder is already decoding that song; if so, it will attempt to reuse
it by seeking it to the new position.  That however fails if the
decoder is not seekable (e.g. a radio stream) which leaves the user
unable to switch to that song with the bogus error message "Not
seekable".

This fixes spurious replay gain logs when the player inserts silence
chunks, because those silence chunks had no replay gain attached,
resetting the ReplayGainFilter state, flipping it forth and back.

This attribute is not particularly important, but it was
uninitialized.

This fixes an assertion failure caused by resuming playback before the
decoder has finished startup.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Eliminate a dependency from MultipleOutputs on PlayerControl.

Obsoletes the same variable from AudioConfig.cxx.

Don't use the global variables in class DecoderBridge; instead,
forward these values to the decoder thread via PlayerControl and
DecoderControl.

Prepare full C++ exception support in the decoder thread.

Prepare full C++ exception support in the player thread.

Fixes assertion failure in MultipleOutputs::Open() after seeking to a
failing song during playback.

Fix regression by commit 45f6129a

See http://bugs.musicpd.org/view.php?id=4477

Instead of stopping playback completely, only CANCEL the queued song
if necessary, and use the SEEK command to play the selected song.
SEEK will take care for current playback state.