Add context menu for current location

Add a new boolean option herecmd_menu. If this is on, and using
a windowport that supports mouse, clicking on your character pops
up a menu of actions doable in that location. Basically this is
nothing new, as almost all of the same actions were done before
on the mouse click.

You can also pop up the context menu with the #herecmdmenu
extended command

include/flag.h

@@ -191,6 +191,7 @@ struct instance_flags {
      * behaviour of various NetHack functions and probably warrant
      * a structure of their own elsewhere some day.
      */
+    boolean herecmd_menu;  /* use menu when mouseclick on yourself */
     boolean invis_goldsym; /* gold symbol is ' '? */
     int parse_config_file_src;  /* hack for parse_config_line() */
     int in_lava_effects;   /* hack for Boots_off() */