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

@@ -79,7 +79,9 @@ themerooms = {
    end,
 
    -- Boulder room
-   function()
+   {
+      mindiff = 4,
+      contents = function()
          des.room({ type = "themed",
                   contents = function(rm)
                      for x = 0, rm.width - 1 do
@@ -95,10 +97,13 @@ themerooms = {
                      end
                   end
          });
-   end,
+      end
+   },
 
    -- Spider nest
-   function()
+   {
+      mindiff = 10,
+      contents = function()
          des.room({ type = "themed",
                   contents = function(rm)
                      for x = 0, rm.width - 1 do
@@ -110,7 +115,8 @@ themerooms = {
                      end
                   end
          });
-   end,
+      end
+   },
 
    -- Trap room
    function()