Dehardcode the monk minetown food shop conversion

Instead of trying to figure out in core whether to change a minetown
food shop to health food shop for monks, just figure it out in the
minetown level creation script.

dat/minetn-4.lua

@@ -69,7 +69,7 @@ des.room({ type = "ordinary", lit=1, x=3,y=3,
                          end
               })
 
-              des.room({ type = "food shop", chance=90, lit=1, x=11, y=11, w=3, h=2,
+              des.room({ type = monkfoodshop(), chance=90, lit=1, x=11, y=11, w=3, h=2,
                          contents = function()
                             des.door({ state = "closed", wall="east" })
                          end