This unifies the two separate special-room-stocking code paths, one in
the standard dungeon generator and one in the special level generator
(neither of which reacted to themed rooms, which is the reason for this
commit) into the end of makelevel(), placing the special room stocking
as the very last step of level creation.
Under the new system, when a regular or special level decides to create
a special room, it sets that room's rtype, but the room is not stocked
until later. It already worked this way for special levels, so the main
difference here is in the normal level generation, where the mkroom
family of functions identifies and marks a room as a special room, but
stops short of filling it. (I suppose perhaps the mkroom, mkzoo, mkshop
family of functions would be better off changing their names to
"pickroom" and so on.)
This also restructures makelevel() itself a bit, but the only real
change is that the paths that call makemaz don't return immediately
afterward; they continue to the special room stocking code. Also, this
code was lifted from fill_special_rooms, which is now not used
anywhere, so it has been deleted.
I don't really like how fill_ordinary_room is in mklev.c and
fill_special_room is in sp_lev.c; they seem like they'd be better off in
mkroom.c, but in the interest of not making unnecessary code changes,
I'll just recommend it.
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