Allow creating unhidden traps in special levels

des.trap({ type = "rust", seen = true });

src/mklev.c

@@ -1468,6 +1468,8 @@ mktrap(int num, int mktrapflags, struct mkroom *croom, coord *tm)
 
     if (kind == WEB && !(mktrapflags & MKTRAP_NOSPIDERONWEB))
         (void) makemon(&mons[PM_GIANT_SPIDER], m.x, m.y, NO_MM_FLAGS);
+    if ((mktrapflags & MKTRAP_SEEN))
+        t->tseen = TRUE;
 
     /* The hero isn't the only person who's entered the dungeon in
        search of treasure. On the very shallowest levels, there's a