two-weapon combat on/off via inventory item action

When testing 'm )' I noticed that weapon and alternate weapon weren't
offering the chance to toggle two-weapon mode.  When already on,
providing it as a choice to toggle it off is simple, but when at is
off that isn't the case.  There are lots of reasons why attempting
to toggle it on might fail and it is silly to offer as a choice if
failure is sure to occur.  This tries to filter out the majority of
reasons why the player can't toggle it on when deciding whether to
include 'X' as a choice.

doc/fixes3-7-0.txt

@@ -1752,6 +1752,7 @@ when using 'm #overview' to annotate a level other than current the one,
 when using 'm #overview' in the endgame, don't include non-endgame levels
 when a tethered aklys gets caught in a web, make sure it doesn't return
 	and that the display of the tether gets cleaned up
+add 'X' as a potential context-sensitive item-action for uwep and uswapwep
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

src/invent.c

@@ -2827,6 +2827,7 @@ enum item_action_actions {
     IA_WIELD_OBJ,
     IA_WEAR_OBJ,
     IA_SWAPWEAPON,
+    IA_TWOWEAPON,
     IA_ZAP_OBJ,
     IA_WHATIS_OBJ, /* '/' specify inventory object */
 };
@@ -3044,6 +3045,9 @@ itemactions_pushkeys(struct obj *otmp, int act)
         case IA_SWAPWEAPON:
             cmdq_add_ec(CQ_CANNED, doswapweapon);
             break;
+        case IA_TWOWEAPON:
+            cmdq_add_ec(CQ_CANNED, dotwoweapon);
+            break;
         case IA_ZAP_OBJ:
             cmdq_add_ec(CQ_CANNED, dozap);
             cmdq_add_key(CQ_CANNED, otmp->invlet);
@@ -3339,6 +3343,30 @@ itemactions(struct obj *otmp)
     else if (otmp == uswapwep)
         ia_addmenu(win, IA_SWAPWEAPON, 'x', "Swap this with your main weapon");
 
+    /* this is based on TWOWEAPOK() in wield.c; we don't call can_two_weapon()
+       because it is very verbose; attempting to two-weapon might be rejected
+       but we screen out most reasons for rejection before offering it as a
+       choice */
+#define MAYBETWOWEAPON(obj) \
+    ((((obj)->oclass == WEAPON_CLASS)                           \
+      ? !(is_launcher(obj) || is_ammo(obj) || is_missile(obj))  \
+      : is_weptool(obj))                                        \
+     && !bimanual(obj))
+
+    /* X: Toggle two-weapon mode on or off */
+    if ((otmp == uwep || otmp == uswapwep)
+        /* if already two-weaponing, no special checks needed to toggle off */
+        && (u.twoweap
+        /* but if not, try to filter most "you can't do that" here */
+            || (could_twoweap(gy.youmonst.data) && !uarms
+                && uwep && MAYBETWOWEAPON(uwep)
+                && uswapwep && MAYBETWOWEAPON(uswapwep)))) {
+        Sprintf(buf, "Toggle two-weapon combat %s", u.twoweap ? "off" : "on");
+        ia_addmenu(win, IA_TWOWEAPON, 'X', buf);
+    }
+
+#undef MAYBETWOWEAPON
+
     /* z: Zap wand */
     if (otmp->oclass == WAND_CLASS)
         ia_addmenu(win, IA_ZAP_OBJ, 'z', "Zap this wand to release its magic");

src/wield.c

@@ -73,7 +73,7 @@ static int wield_ok(struct obj *);
    empty hands and two-handed weapons have to be handled separately */
 #define TWOWEAPOK(obj) \
     (((obj)->oclass == WEAPON_CLASS)                            \
-     ? !(is_launcher(obj) ||is_ammo(obj) || is_missile(obj))    \
+     ? !(is_launcher(obj) || is_ammo(obj) || is_missile(obj))   \
      : is_weptool(obj))
 
 static const char