Add support for link status, media and VLAN MTU (if supported) to if_cdce...

This makes it more usable in that dhclient will autolaunch from devd
now when cdce devices are plugged in.. It also sets the baudrate, but
this isn't exported via tools, and CDCE doesn't have a good way to
specify the media type, so there isn't a good way to tell userland
what the speed is currently...

Reviewed by: hps
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D30625