Fix monsters not wielding digging implements

My change to unify the pet and monster digging weapon choosing
made monsters not actually wield the chosen weapon, even though
they could still dig as if they did. Pets behaved properly.

Also add an explicit check for IS_STWALL so they won't keep
switching away from their current weapon until needed.

doc/fixes36.2

@@ -99,6 +99,7 @@ prevent wish prompt input from remembering the previous wish
 parchment and vellum are made from animal skin so change material composition
 	and color for spellbooks with those descriptions from paper to leather;
 	eating those books now breaks vegetarian conduct
+fix monsters not wielding digging implements
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository