Accessibility: Pick travel/cursor targets from a menu

Adds two new configurable keys to the cursor targeting: 'A' (getpos.menu)
and 'a' (getpos.menu.cansee). First one shows a menu of all interesting
glyphs on the map, second one shows only those in sight.

Travel command also now obeys the "request menu" -prefix, showing
the menu with interesting targets in sight, and then traveling there.

Idea via the NetHack accessibility research by Alexei Pepers.

include/flag.h

@@ -468,6 +468,8 @@ enum nh_keyfunc {
     NHKF_GETPOS_UNEX_NEXT,
     NHKF_GETPOS_UNEX_PREV,
     NHKF_GETPOS_HELP,
+    NHKF_GETPOS_MENU,
+    NHKF_GETPOS_MENU_FOV,
 
     NUM_NHKF
 };