Minor fixes to menus on XI2

Minor fixes to menus on XI2

* src/xfns.c (Fx_create_frame): Populate `xi_masks'.
* src/xmenu.c (x_activate_menubar)
(create_and_show_popup_menu, x_menu_show): Only clear input
extension grabs if we (or the toolkit) actually selected for
XI_ButtonPress events.
* src/xterm.c (xi_frame_selected_for): New function.
(xi_populate_device_from_info, handle_one_xevent): Store device
use instead of just whether or not it's a master device.
(x_dnd_begin_drag_and_drop): Clean up block_input stuff.
* src/xterm.h: Update prototypes.
(struct xi_device_t): Rename `master_p' to `use'.