Add more explicit helpless reasons

Instead of just "while helpless", the death reason will tell
more explicitly why the player was helpless.  For example:
"while frozen by a monster's gaze"

src/trap.c

@@ -4447,6 +4447,7 @@ boolean disarm;
 			if (!Free_action) {                        
 			pline("Suddenly you are frozen in place!");
 			nomul(-d(5, 6));
+			multi_reason = "frozen by a trap";
 			exercise(A_DEX, FALSE);
 			nomovemsg = You_can_move_again;
 			} else You("momentarily stiffen.");