inventory item-action: add '-' for uswapwep

There's no 'w-' or 'Q-' for alternate weapon, but context-sensitive
inventory is starting from the object rather than the command so can
finesse that.  'A' does allow alternate weapon to be directly unset
(aka reset to bare/gloved hands) but is not friendly to being passed
queued input.

This adds an extra internal command which only handles unset uswapwep,
even though that is something which is awfully specialized to get it's
own command.  Users don't see this command so that shouldn't matter.

src/cmd.c

@@ -2606,6 +2606,7 @@ struct ext_func_tab extcmdlist[] = {
     { '\0', "clicklook", NULL, doclicklook, INTERNALCMD, NULL },
     { '\0', "altdip", NULL, dip_into, INTERNALCMD, NULL },
     { '\0', "altadjust", NULL, adjust_split, INTERNALCMD, NULL },
+    { '\0', "altunwield", NULL, remarm_swapwep, INTERNALCMD, NULL },
     { '\0', (char *) 0, (char *) 0, donull, 0, (char *) 0 } /* sentinel */
 };