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.

Another big chunk of code for multi-player support.