* src/keyboard.c (timer_start_idle): Call internal-timer-start-idle instead

* src/keyboard.c (timer_start_idle): Call internal-timer-start-idle instead
of marking the idle timers directly.
* lisp/emacs-lisp/timer.el (timer--check): New function.
(timer--time, timer-set-function, timer-event-handler): Use it.
(timer-set-idle-time): Simplify.
(timer--activate): CSE.
(timer-event-handler): Give more info in error message.
(internal-timer-start-idle): New function, moved from C.