guarantee PV extension for nodestime and handle MultiPV
This PR fixes two minor issues with master's PV extension code.
1. When 'nodestime' is active, master may abort PV extension if it takes too long (as measured in ms), rendering the produced results nondeterministic. The patch always completes the PV extensions. This has no effect on playing strength because the local calls to `pos.do_move()` do not update the global used nodes counter.
2. If standard time management is active, and the game is played with `multiPV > 1`, then extending several PVs may repeatedly use up to `moveOverhead / 2` ms, possibly leading to time losses. The patch fixes this by only extending the PV of the best move during game play.
closes https://github.com/official-stockfish/Stockfish/pull/7079
No functional change