lua/playlist: do not handle impossible error
playlist: remove unused return value from playlist_Control()
playlist: rename playlist_Pause() to playlist_TogglePause()

The behaviour is unchanged:
 - if stopped: starts playing,
 - if playing: pauses playing,
 - if paused: resumes playing.
playlist: remove request.i_status
playlist: remove status.i_status
playlist: add (proper) playlist_Pause() and playlist_Resume()

Those two functions have no effects if the playlist is stopped.
Otherwise they force the playlist to playing ("running") or paused
state respectively.

As a reminder, the existing playlist_Play() forces the playlist into
running state (unless it is empty), and playlist_Stop() forces the
playlist into stopped state (unless the input thread refuses to die).
There are no functions to force the playlist to paused state.
playlist: use playlist_Play() directly, fix minor races

This function already checks if the playlist is running internally,
in which case it has no effects.
playlist: use playlist_TogglePause() directly, fix minor races

This function already starts playback if stopped (and as far as I know
it always has).
playlist: use the new playlist_Pause() where appropriate

This fixes corking especially.
playlist: automatically resume from pause when uncorked