Fix github issue #1016 - buried zombie theme room

Reported by vultur-cadens:  it was possible to encounter ettin zombies
and giant zombies at shallow levels if those levels were generated
with the "buried zombies" theme room.

This uses a simpler fix than the suggestion(s); it's only applicable
to the zombie room though.

Also, vary the zombify_mon timeout.  If the hero happened to be in
the room at the time that that expired, they would all claw their way
out of the ground at once.  The new variation is quite small so they
still escape burial as a group, but over the span of 21 turns rather
than all on one turn.

Fixes #1016

dat/themerms.lua

@@ -1,6 +1,7 @@
 -- NetHack themerms.lua	$NHDT-Date: 1652196294 2022/05/10 15:24:54 $  $NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.16 $
 --	Copyright (c) 2020 by Pasi Kallinen
 -- NetHack may be freely redistributed.  See license for details.
+--
 -- themerooms is an array of tables and/or functions.
 -- the tables define "frequency", "contents", "mindiff" and "maxdiff".
 -- frequency is optional; if omitted, 1 is assumed.
@@ -95,12 +96,21 @@ themeroom_fills = {
 
    -- Buried zombies
    function(rm)
-      local zombifiable = { "kobold", "gnome", "orc", "dwarf", "elf", "human", "ettin", "giant" };
+      local diff = nh.level_difficulty()
+      -- start with [1..4] for low difficulty
+      local zombifiable = { "kobold", "gnome", "orc", "dwarf" };
+      if diff > 3 then          -- medium difficulty
+         zombifiable[5], zombifiable[6] = "elf", "human";
+         if diff > 6 then       -- high difficulty (relatively speaking)
+            zombifiable[7], zombifiable[8] = "ettin", "giant";
+         end
+      end
       for i = 1, (rm.width * rm.height) / 2 do
          shuffle(zombifiable);
-         local o = des.object({ id = "corpse", montype = zombifiable[1], buried = true });
+         local o = des.object({ id = "corpse", montype = zombifiable[1],
+                                buried = true });
          o:stop_timer("rot-corpse");
-         o:start_timer("zombify-mon", 1000);
+         o:start_timer("zombify-mon", math.random(990, 1010));
       end
    end,
 
@@ -265,6 +275,7 @@ themerooms = {
                  end
       });
    end,
+
    {
       frequency = 6,
       contents = function()