Make using quiescence search implicit
If search depth is less than ONE_PLY call qsearch(), no need to check the
depth condition at various call sites of search().
Passed STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 14568 W: 3011 L: 2877 D: 8680
http://tests.stockfishchess.org/tests/view/5aa846190ebc59029781015b
Also helps gcc to find some optimizations (smaller binary, some speedup).
Thanks to Aram and Stefan for identifying an oversight in an early version.
Closes https://github.com/official-stockfish/Stockfish/pull/1487
No functional change.