B02001 - trees and monster

<Someone> noticed that the change to require axes for trees (and allow them for
doors) did not extend to monsters.  Now it does.
- added 2 new weapon check flags to handle the new cases
- added some detailed digging flags to mfndpos, based on ALLOW_DIG, and
  moved some common logic regarding that flag into mfndpos
- made the ARMS check consistent for 2-handed weapons
I also noticed that simply carrying a pick was enough to allow a monster to
dig a door; wielding wasn't required.  This is fixed as well.

include/monst.h

@@ -6,7 +6,7 @@
 #define MONST_H
 
 /* The weapon_check flag is used two ways:
- * 1) When calling mon_wield_item, is 2, 3, or 4 depending on what is desired.
+ * 1) When calling mon_wield_item, is 2-6 depending on what is desired.
  * 2) Between calls to mon_wield_item, is 0 or 1 depending on whether or not
  *    the weapon is known by the monster to be cursed (so it shouldn't bother
  *    trying for another weapon).
@@ -20,6 +20,8 @@
 # define NEED_RANGED_WEAPON 2
 # define NEED_HTH_WEAPON 3
 # define NEED_PICK_AXE 4
+# define NEED_AXE 5
+# define NEED_PICK_OR_AXE 6
 
 /* The following flags are used for the second argument to display_minventory
  * in invent.c: