Clean up some uses of XInternAtom

Clean up some uses of XInternAtom

* src/xfns.c (x_set_undecorated, x_set_no_focus_on_map, x_window)
(set_machine_and_pid_properties): Move calls to XInternAtom for
static string to use previously interned atoms.
(Fx_change_window_property): Use XCB if available to avoid extra
call to XSync.
* src/xterm.c (x_term_init):
* src/xterm.h (struct x_display_info): New atoms _MOTIF_WM_HINTS
and _NET_WM_PID.