Change command X to twoweapon toggle

Explore mode is now an extended command #exploremode.
There's no sense that a command used max. once per game, and
in normal games not at all, takes up a key. So, analogous to
the 'x' command (swap weapons), 'X' now toggles two-weapon
combat.

dat/help

@@ -147,7 +147,7 @@ Commands:
         w       Wield weapon.  w- means wield nothing, use bare hands.
         W       Wear armor.
         x       Swap wielded and secondary weapons.
-        X       Switch the game to explore (discovery) mode.
+        X       Toggle two-weapon combat.
         ^X      Show your attributes.
         z       Zap a wand.  (Use y instead of z if number_pad is -1.)
         Z       Cast a spell.  (Use Y instead of Z if number_pad is -1.)