Restart blink cursor timers on interval changes

Restart blink cursor timers on interval changes

This prevents surprising behavior when timer interval customizations are
only applied whenever the timers happen to be restarted (see Bug#24372).

* lisp/frame.el (blink-cursor--start-idle-timer)
(blink-cursor--start-timer): New functions.
(blink-cursor-start, blink-cursor-check, blink-cursor-mode): Use
the new helper functions.
(blink-cursor-delay, blink-cursor-interval): Restart timers when
the value is changed.