reimplement pull request #944 - grave contents

Pull request from entrez:  if bones left dead hero's corpse on top
of a new grave, don't find a corpse or summon a zombie when digging
the grave up.  It also removed the chance that a ghoul might be
summoned when engraving on a headstone, switching to zombie or mummy
instead.

Rather than adopting the pull request, this retains summoning a
ghoul via engraving and adds the possibly of doing so when kicking
a headstone.  Having a ghoul prowl around the grave is independent
of whether there is a corpse or zombie inside the grave.  To achieve
this, another flag in 'struct rm' is needed; the single bit for
'disturbed' isn't sufficient.  The bigger 'flags' field wasn't in
use for graves so commandeer that for new 'emptygrave'.  'disturbed'
still uses the 'horizontal' bit in order to have engraving and/or
kicking summon at most one ghoul.

Closes #944

include/extern.h

@@ -828,6 +828,7 @@ extern void engr_stats(const char *, char *, long *, long *);
 extern void del_engr(struct engr *);
 extern void rloc_engr(struct engr *);
 extern void make_grave(coordxy, coordxy, const char *);
+extern void disturb_grave(coordxy, coordxy);
 
 /* ### exper.c ### */