dungeon_overview adjustment

The dungeon_overview bits in the rm structure were being
clobbered by a run-length encoding save/restore because
they weren't taken into consideration.

This patch pulls that data out of the rm structure completely.

It also adjusts the run-length encoding checks to take the
candig bit into consideration and adds a comment to rm.h
reminding people to make run-length encoding adjustments
in save.c for any new bits that get added.

src/read.c

@@ -573,7 +573,7 @@ forget_map(howmuch)
 		levl[zx][zy].waslit = 0;
 		levl[zx][zy].glyph = cmap_to_glyph(S_stone);
 #ifdef DUNGEON_OVERVIEW
-		levl[zx][zy].styp = STONE;
+		lastseentyp[zx][zy] = STONE;
 #endif
 	    }
 }