Fix termination sequence on Windows wrt interval timers.

Fix termination sequence on Windows wrt interval timers.

 src/w32proc.c <disable_itimers>: New static flag.
 (init_timers): Initialize it to zero, after creating the critical
 sections used by the timer threads.
 (term_timers): Set to 1 before deleting the critical sections.
 (getitimer, setitimer): If disable_itimers is non-zero, return an
 error indication without doing anything.  Reported by Fabrice
 Popineau <fabrice.popineau@supelec.fr> as part of bug#12544.