Allow matching any wall map terrain in lua scripts

When matching a terrain, allow using a "w" placeholder that matches
any solid wall:

For example:
   local s = selection.match([[w.w]]);
would match all floor locations with a wall to the left and right of it.
The walls can be solid stone, horizontal, vertical, etc.

This applies to selection.match(), selection.filter_mapchar(), and
des.replace_terrain()

include/rm.h

@@ -74,6 +74,7 @@ enum levl_typ_types {
     CLOUD     = 35,
 
     MAX_TYPE  = 36,
+    MATCH_WALL = 37,
     INVALID_TYPE = 127
 };