adding if_dev member to struct ifnet

[Vincent Jardin]

> I've done something like this in my perforce branch
> (//depot/user/brooks/xname/...), but it isn't a real solution because
> I plan to follow up by breaking the assumption that if_xname remains
> constant for the life of the interface.  If either if_dev or
> if_d{name,unit} are added, you could use those as you do now.
> You might consider using if_index instead since that's both unchanged
> over the life of the device and quick to check.

In this case, I agree that using if_index would be a better choice ;-)

Regards,
  Vincent