Modify preemptive redisplay to be based on periodic checks for input.

Modify preemptive redisplay to be based on periodic checks for input.
(PERIODIC_PREEMPTION_CHECKING): Define to 1 iff EMACS_HAS_USECS.
(Vredisplay_preemption_period): New variable.
(syms_of_display): DEFVAR_LISP and initialize it.
(preemption_period, preemption_next_check): New variables.
(update_frame, update_single_window): Initialize them based on
Vredisplay_preemption_period if !force_p.
(update_window, update_frame_1): Use them to determine when to
check for input.