Added the ability to:
* Create virtual interfaces
* Create vlan interfaces
* Get interface fib
* Get interface groups
* Get interface status
* Get nd6 info
* Get media status
* Get additional ifaddr info in a convenient struct
* Get vhids
* Get carp info
* Get lagg and laggport status
* Iterate over all interfaces and ifaddrs
And add more examples, too.
Note that this is a backwards-incompatible change. But that's ok, because it's
a private library.
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D14463
