Implement EGBB probing and -first/secondDrawDepth

Implement EGBB probing and -first/secondDrawDepth

The new volatile options -first/secondDrawDepth N can be used with engines
that use tablebase, to limit their search depth to N ply in drawn positions
(as determined by bitbase probing). This prevents them from wasting lots
of CPU time on deep searches, when we know there is nothing to be discovered.
In won or lost positios such engines should move instantly anyway.