Add const qualifer to check_is_dangerous

No functional change.
Rename posKey stored in the transposition table

[Edit: Slightly extended by me]

No functional change.
Change slave_available() API

To return a pointer to the available
thread instead of a bool. This allows
to simplify the core loop in split().

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Add KBPKP endgame

It is a draw if pawns are on G or B files, weaker pawn is
on rank 7 and bishop can't attack the pawn.

No functional change (because it is very rare and does not appear in bench)
More UCI cleanup

Fix debug on in debug mode
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/tt.cpp