monitor: Fix check process about multiple starts of sssd when pidfile remains
    
    If PIDFile is invalid in sssd.service, pidfile remains if sssd terminates abnormally.
    Also, if /var/run is not tmpfs, the pidfile will remain when the OS is forcibly stopped.
    
    In check process about multiple starts of sssd, only the existence of pidfile is checked.
    Fix not only to check if pidfile exists, but also to check if PID exists.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>