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.

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.

Assigning nullptr to next_song may disrupt the player thread and
render undefined behavior.

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