More behind the scenes transparent speedups around xselect.c

More behind the scenes transparent speedups around xselect.c

* src/xdisp.c (display_menu_bar): Fix compiler warning about
NULL pointer dereference.
* src/xfns.c (Fx_begin_drag): Use x_intern_atoms.
(Fx_change_window_property): Pass dpyinfo to
x_fill_property_data.
* src/xselect.c (lisp_data_to_selection_data): Use
x_intern_atoms instead of syncing for each atom.
(x_fill_property_data, x_send_client_event): Use
x_intern_cached_atom.
* src/xterm.c (x_intern_atoms): New function.
* src/xterm.h: Update prototypes.