Add minimum difficulty cutoffs to two themed rooms

This sets the minimum level depth of "Spider nest" to 10, somewhat above
the difficulty of an individual giant spider, because a whole room full
of them is a tougher challenge. Note that this isn't the only possible
fix to this problem; another solution would be to alter the special case
in mktrap that hardcodes a giant spider to generate with each web to
produce cave spiders if giant spiders would be too tough. Even then, a
lower difficulty cutoff is probably still warranted for this room, since
a large number of cave spiders might be too tough for level 1 or 2.

This also sets the minimum level depth of "Boulder room" to 4, based on
the fact that individual rolling boulder traps normally can't appear
until level 2, and having a bunch of them in one place which may be
required to reach the downstairs could be problematic.

This doesn't do anything to address the "Mausoleum" room problem, in
which a master or arch-lich can generate and immediately warp out and
attack the player. Even with a high difficulty threshold, it won't fix
the problem of these liches generating out of their normal difficulty
and Gehennom constraints.

Other potential candidates for difficulty thresholds:
 - "Trap room": This room might be perilous on the first few levels,
   especially if the level generates with it blocking the way to the
   downstairs.
 - "Massacre": Doesn't have any particular hazards, but might be
   interesting if it only generated at deeper levels.

dat/themerms.lua

@@ -16,7 +16,7 @@
 -- to generate a single themed room.
 
 themerooms = {
-  {
+   {
      -- the "default" room
       frequency = 1000,
       contents = function()
@@ -79,38 +79,44 @@ themerooms = {
    end,
 
    -- Boulder room
-   function()
+   {
-      des.room({ type = "themed",
+      mindiff = 4,
-                 contents = function(rm)
+      contents = function()
-                    for x = 0, rm.width - 1 do
+         des.room({ type = "themed",
-                       for y = 0, rm.height - 1 do
+                  contents = function(rm)
-                          if (percent(30)) then
+                     for x = 0, rm.width - 1 do
-                             if (percent(50)) then
+                        for y = 0, rm.height - 1 do
-                                des.object("boulder");
+                           if (percent(30)) then
-                             else
+                              if (percent(50)) then
-                                des.trap("rolling boulder");
+                                 des.object("boulder");
-                             end
+                              else
-                          end
+                                 des.trap("rolling boulder");
-                       end
+                              end
-                    end
+                           end
-                 end
+                        end
-      });
+                     end
-   end,
+                  end
+         });
+      end
+   },
 
    -- Spider nest
-   function()
+   {
-      des.room({ type = "themed",
+      mindiff = 10,
-                 contents = function(rm)
+      contents = function()
-                    for x = 0, rm.width - 1 do
+         des.room({ type = "themed",
-                       for y = 0, rm.height - 1 do
+                  contents = function(rm)
-                          if (percent(30)) then
+                     for x = 0, rm.width - 1 do
-                             des.trap("web", x, y);
+                        for y = 0, rm.height - 1 do
-                          end
+                           if (percent(30)) then
-                       end
+                              des.trap("web", x, y);
-                    end
+                           end
-                 end
+                        end
-      });
+                     end
-   end,
+                  end
+         });
+      end
+   },
 
    -- Trap room
    function()