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.

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

If the decoder was just started, it already seeks to the desired
position.

If this gets called while seeking, do the right thing.  This moves the
elapsed_time setter from method Run(), which is now redundant.

Allows removing a few special cases in the player thread.

It is futile to check for decoder errors before the decoder has
finished startup.  At this time, it's unlikely that the decoder has
already failed.

.. and fix its API documentation.

Move ResetCrossFade() to where cross-fading would actually break, to
make the cross-fading state more reliable.

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