Fix bugs #12447 and #12326 with infloop causes by idle timers, update docs.
src/keyboard.c (timer_check_2): Move calculation of 'timers' and
'idle_timers' from here ...
(timer_check): ... to here. Use Fcopy_sequence to copy the timer
lists, to avoid infloops when the timer does something stupid,
like reinvoke itself with the same or smaller time-out.
lisp/emacs-lisp/timer.el (run-with-idle-timer)
(timer-activate-when-idle): Warn against reinvoking an idle timer
from within its own timer action.
doc/lispref/os.texi (Idle Timers): Warn against reinvoking an idle timer
from within its own timer action.