howmonseen - monster visibility (trunk only)

Pull some code out of lookat() so that it can be used elsewhere.
howmonseen(mon) returns a bitmask of the ways that hero can see mon.

include/extern.h

@@ -2402,6 +2402,7 @@ E void FDECL(unblock_point, (int,int));
 E boolean FDECL(clear_path, (int,int,int,int));
 E void FDECL(do_clear_area, (int,int,int,
 			     void (*)(int,int,genericptr_t),genericptr_t));
+E unsigned FDECL(howmonseen, (struct monst *));
 
 #ifdef VMS