Fix loading of opening line

Fix loading of opening line

Appending moves to the game out of sync with otheer commands (i.e. by
the GUI thread) did not work properly: XBoard can send 'new', 'force'
plus a ton of moves all at once, and the appending of moves would then
overtake the 'new'  command (first queued for the engine thread), which
would clear the move number, deleting the moves again. Now the appending
of moves is also deferred to the engine thread, by queuing them.