Send game result to USI engines

Send game result to USI engines

The USI 'gameover' command is now used as translation of the CECP
'result' command, to relay the result to the engine. To decide how
the absolute game result (white/black) is translated to a relative one
(win/lose), the side last moved for is used as indication, and remembered
whenever the engine is set thinking on a move (as it could already be
switched to force mode at the time the 'result' command is received).