Augment struct mkroom with a short string (perhaps 3 or 8 characters). The purpose of this string is to store a unique identifier for non-special types of room, in particular different themed room types. This can then be used later in the game (e.g. for printing an entry message in certain themed rooms) without interfering with the actual room types as far as level generation is concerned.

This uses a string identifier mainly because that makes it straightforward to initialize the string in Lua, versus a numeric constant the name of which would not be exposed to Lua.