wdmd: prevent probe while watchdog is used
    
    If the wdmd daemon is killed uncleanly, /dev/watchdog
    is closed uncleanly and the watchdog should fire to
    reset the machine.
    
    If 'wdmd -p' is run to probe available watchdog devices
    after wdmd is killed and before the machine is reset,
    it will unintentionally interfere with the expiring
    watchdog by doing open/magic-close on each device.
    This silently prevents the watchdog from firing when
    it should.
    
    The probe function can check for the shm flag before
    touching any watchdog devices, just like another wdmd
    daemon process does, to avoid interfering with the
    watchdog state of a previous wdmd daemon that was
    terminated uncleanly.