half MATE value
idea is to save one iteration of aspiration window, when we are finding a mate. makes no difference on the bench, and most likely no measurable ELO difference, but should allow to find mates a little faster.
clarify MAX_PLY
and also bug fix mate scores while at it. for mate scores we need to use MAX_PLY which is the total of max search + qsearch plies.

also remove the memcpy and do a manual loop for PV backup.