Level temperature

Allow setting a per-level "temperature": hot, cold, or temperate
via special level flags. Currently it only affects some messages
in Gehennom, but it could be expanded to ice melting, water freezing,
or monster generation, for example.

Invalidates saves and bones.

include/rm.h

@@ -386,6 +386,7 @@ struct levelflags {
                                   normal mode descendant of such) */
     Bitfield(corrmaze, 1);     /* Whether corridors are used for the maze
                                   rather than ROOM */
+    schar temperature;         /* +1 == hot, -1 == cold */
 };
 
 typedef struct {