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/dig.c

@@ -1023,6 +1023,7 @@ struct obj *obj;
 			    /* you ought to be able to let go; tough luck */
 			    /* (maybe `move_into_trap()' would be better) */
 			    nomul(-d(2,2));
+			    multi_reason = "stuck in a spider web";
 			    nomovemsg = "You pull free.";
 			} else if (lev->typ == IRONBARS) {
 			    pline("Clang!");