fix #H2642 - escaping from drowning by moving diagonally

From a bug report, you could crawl
out of water to avoid drowning by moving diagonally into an intact doorway
even though regular movement won't allow that.  (Second version of Medusa's
level has a door adjacent to water.)  You could also escape diagonally when
polymorphed into a grid bug.

include/extern.h

@@ -810,6 +810,7 @@ E boolean FDECL(in_town, (int,int));
 E void FDECL(check_special_room, (BOOLEAN_P));
 E int NDECL(dopickup);
 E void NDECL(lookaround);
+E boolean FDECL(crawl_destination, (int,int));
 E int NDECL(monster_nearby);
 E void FDECL(nomul, (int));
 E void FDECL(unmul, (const char *));