RFT: improvements to if_cdce driver

[John-Mark Gurney]

Hello,

I decided to make some improvements to the CDCE driver as at least
the RealTek devices (what I tested them with) when they aren't supported
by ure will present as cdce devices.

https://reviews.freebsd.org/D30625

This adds if_media support and link state support.

The most significant change is this means that if a ue device is
configured for DHCP, devd will now launch dhclient, where previously
it would not, as it would neither receive the link up status (for when
a cable is plugged in) nor would it be the requisit ethernet media
type.

The device I tested with was a RealTek 2.5G device. So, other
non-RealTek devices would be great to test with.

Let me know if you have any issues with the change!

Thanks.