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-3.lua

@@ -84,7 +84,7 @@ des.room({ type = "ordinary",lit=1,x=3,y=3,
              end
   })
 
-  des.room({ type = "food shop", chance=90,lit=1,x=26,y=8,w=3,h=2,
+  des.room({ type = monkfoodshop(), chance=90,lit=1,x=26,y=8,w=3,h=2,
              contents = function()
                 des.door({ state="closed", wall="west" })
              end