steeds don't remember traps

<Someone> reported to the list that steeds didn't remember traps
encountered while mounted.  When not mounted, a monster will remember
traps, even when they don't damage the monster.  To that end, added code to
set the steed's mtrapseen mask.

src/trap.c

@@ -552,6 +552,10 @@ unsigned trflags;
 	    }
 	}
 
+#ifdef STEED
+	if (u.usteed) u.usteed->mtrapseen |= (1 << (ttype-1));
+#endif
+
 	switch(ttype) {
 	    case ARROW_TRAP:
 		seetrap(trap);