Don't prune discovered checks

Don't prune and eventually extend check moves of type
DISCO_CHECK (pun intended, Lucas will understand :-) ).

Patch from Lucas Braesch that has also tested it:

Result: 879-661-2137, score=52.96%, LOS=100.00% (time 10"+0.1")

I have started a verification test right now.

bench: 6004966

src/types.h

@@ -130,6 +130,12 @@ enum MoveType {
   CASTLE    = 3 << 14
 };
 
+enum CheckType {
+  NO_CHECK,
+  DIRECT_CHECK,
+  DISCO_CHECK
+};
+
 enum CastleRight {  // Defined as in PolyGlot book hash key
   CASTLES_NONE = 0,
   WHITE_OO     = 1,