libifconfig: multiple feature additions

[asomers]

Summary

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
• Add functions for looping over all interfaces and ifaddrs
• Add more examples

Note that this is a backwards-incompatible change. But that's ok, because it's
a private library.

Test Plan

Running at $WORK with proprietary client

[asomers]

Due to time constraints on my end, I intend to commit within 48 hours unless a reviewer raises major issues.

[asomers]

For the record, this PR is based on the content at https://github.com/Savagedlight/libifconfig/pull/46 . IMHO, all new libifconfig development should continue in SVN in the FreeBSD tree.

[jpaetzel]

For that to happen the original author will need a commit bit, which I'll be more than happy to mentor.