crossing long worms' tails (trunk only)

This is one of the items from "#Q397: List of Bugs from #nethack" sent
in Janurary by <email deleted> and containing a list
of things collected from the IRC channel associated with nethack.alt.org's
public server.  Moving diagonally between segments of a worm tail is
conceptually passing right through the worm's body.  This patch prevents
moving in such a fashion for both the hero and monsters (it's still
possible to fight in that position though).  It only applies when the two
tail segments are consecutive.

|......  In the diagram here, where tail segments are represented by
|.w1?..  digits indicating relative sequence number, the @ can still
|..@2..  move between segments 2 and 5 to reach !, but can no longer
|.65!3.  move between 1 and 2 to reach ?.  [However, if there is a
|...4..  monster at the ? spot, it can still hit @ and vice versa.]

     Missiles and wand zaps still pass through such diagonals without
noticing or affecting the worm.  I'm not sure whether this ought to be
extended to change that--it might get pretty messy since it would need
to be considered during monsters' targetting as well as during the path
traversal itself.

include/extern.h

@@ -2588,6 +2588,7 @@ E void FDECL(remove_worm, (struct monst *));
 E void FDECL(place_worm_tail_randomly, (struct monst *,XCHAR_P,XCHAR_P));
 E int FDECL(count_wsegs, (struct monst *));
 E boolean FDECL(worm_known, (struct monst *));
+E boolean FDECL(worm_cross, (int,int,int,int));
 
 /* ### worn.c ### */