Improve matefinding
When seeking a mate, focus the search on forcing lines, with three search modifications that are guarded by the seekmate condition. On the matetrack suite this finds 4065 mates vs 3898 for master (+167), Measured in isolation the three terms contribute:
contribute +71 (null move reduction), +44 (small ProbCut) and +39 (LMR). A forth term tested in SPRT was dropped as there was no real gain on current master.
As done previously SPRT test vs a control build with seekMate disabled, to guarantee no drift in performance in game play.
passed STC:
LLR: 2.99 (-2.94,2.94) <-1.75,0.25>
Total: 356416 W: 90683 L: 90801 D: 174932
Ptnml(0-2): 429, 37985, 101469, 37925, 400
https://tests.stockfishchess.org/tests/view/6ac166745349156614d93802
passed LTC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 131064 W: 33890 L: 33789 D: 63385
Ptnml(0-2): 14, 12699, 40019, 12772, 28
https://tests.stockfishchess.org/tests/view/6ac5b99e5349156614d93d60
closes https://github.com/official-stockfish/Stockfish/pull/7216
Bench: 1684318