libifconfig: multiple feature additions

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