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.

dat/air.lua

@@ -6,7 +6,7 @@
 --
 des.level_init({ style = "solidfill", fg = " " });
 
-des.level_flags("mazelevel", "noteleport", "hardfloor", "shortsighted")
+des.level_flags("mazelevel", "noteleport", "hardfloor", "shortsighted", "stormy")
 -- The following messages are somewhat obtuse, to make then
 -- equally meaningful if the player can see or not.
 des.message("What a strange feeling!")

doc/lua.adoc

@@ -615,6 +615,7 @@ Set flags for this level.
 | nomongen      | Prevents random monster generation.
 | nodeathdrops  | Prevents killed monsters from dropping corpses or random death drops.
 | fumaroles     | Lava emits poison gas clouds at random.
+| stormy        | Clouds create lightning bolts at random.
 |===
 
 Example:

include/patchlevel.h

@@ -17,7 +17,7 @@
  * Incrementing EDITLEVEL can be used to force invalidation of old bones
  * and save files.
  */
-#define EDITLEVEL 79
+#define EDITLEVEL 80
 
 /*
  * Development status possibilities.

include/rm.h

@@ -393,6 +393,7 @@ struct levelflags {
     Bitfield(deathdrops, 1);   /* monsters may drop corpses/death drops */
     Bitfield(noautosearch, 1); /* automatic searching disabled */
     Bitfield(fumaroles, 1);    /* lava emits poison gas at random */
+    Bitfield(stormy, 1);       /* clouds create lightning bolts at random */
 
     schar temperature;         /* +1 == hot, -1 == cold */
 };

src/sp_lev.c

@@ -3749,6 +3749,8 @@ lspo_level_flags(lua_State *L)
             gl.level.flags.noautosearch = 1;
         else if (!strcmpi(s, "fumaroles"))
             gl.level.flags.fumaroles = 1;
+        else if (!strcmpi(s, "stormy"))
+            gl.level.flags.stormy = 1;
         else {
             char buf[BUFSZ];
             Sprintf(buf, "Unknown level flag %s", s);

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) */