libbsd: React to link status change events

[Jens Gollasch]

> is there an easy way to register a callback function or block until a link status change of an Ethernet device occurs in the libbsd stack?
> Searching for it, I found some references for EV_NETDEV in kqueue, but this seems to be a legacy function which has been removed years ago.
>
>
I solved this with setting an hook function for the dhcpcd and react to 
the "reason" string in the environment.
Link is "CARRIER" and NOCARRIER".
Look at dhcpcd01 test.

-- 
Mit freundlichen Grüßen
Jens Gollasch

Licht-, Steuer- und Schaltanlagenbau GmbH
NL Dresden