fix pull request #365 - monster max HP

The recently added sanity check for monster maximum HP was giving
false complaints when Nd8 monster had N mhpmax.  Most noticeable
for level 1 monsters (level 0 monsters use 1d4 instead of 0d8 and
weren't affected) but possible for higher level ones if they were
unlucky--from their own perspective--with all their d8 rolls.

Give level N monsters a minimum of N+1 HP, so minimum of 2 for
level 1 monsters, making 1/8 of those stronger.  Same minimum for
level 0 monsters, 25% of which will become stronger now.  (The pull
request's patch gave every Nd8 monster 1 extra HP; this only does
so for Nd8 and 1d4 ones which have rolled lowest possible amount.)

Also relax the sanity check so that existing to-be-3.7 save files
don't continue to trigger sanity complaints for existing monsters
that have the old minimum.

Fixes 365

doc/fixes37.0

@@ -1,4 +1,4 @@
-$NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.248 $ $NHDT-Date: 1594730609 2020/07/14 12:43:29 $
+$NHDT-Branch: NetHack-3.7 $:$NHDT-Revision: 1.249 $ $NHDT-Date: 1594771373 2020/07/15 00:02:53 $
 
 General Fixes and Modified Features
 -----------------------------------
@@ -287,6 +287,8 @@ only generate shop items on solid floor squares
 avoid gcc 10 warning by removing duplicate definition of 'head_engr'
 if a monster removed a corpse from an ice box, the corpse would never rot away
 monster creation on quest levels could make genocided creatures
+enabling wizard mode 'sanity_check' option would complain about invalid mhpmax
+	value for level N monsters created with a d8 value of 1 for all N d8's
 
 tty: redraw unexplored locations as S_unexplored rather than <space> after
 	map has been partially overwritten by popup menu or text display