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

doc/fixes3-7-0.txt

@@ -1094,6 +1094,9 @@ demon gating happens more in Gehennom and less outside it
 intelligent peacefuls avoid digging shop or temple walls
 fix bug making random subrooms never touching the right or bottom
 	wall of the parent room
+if a grave is created with the corpse lying on top (bones), don't find a
+	corpse or release a zombie or mummy when digging it up
+kicking a headstone might summon a ghoul
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository