If interface is getting down by netifd (unplug ethernet cable)
mwan3track will not recognize this change. It will also generate an
additional down event when he notice does his tracking interface is offline.

Mwan3track will now be informed by a signal (trap) USR1 during down event
that the interface is already down. An additional down event will not be
generated.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>