Add stormy level flag

Which makes clouds create lightning bolts at random.
This flag is used on the plane of air.

Breaks saves and bones.

src/timeout.c

@@ -1706,8 +1706,8 @@ do_storms(void)
     int dirx, diry;
     int count;
 
-    /* no lightning if not the air level or too often, even then */
-    if (!Is_airlevel(&u.uz) || rn2(8))
+    /* no lightning if not stormy level or too often, even then */
+    if (!gl.level.flags.stormy || rn2(8))
         return;
 
     /* the number of strikes is 8-log2(nstrike) */