user networking for bhyve

Hello,

I implemented a network backend for bhyve which uses libslirp to provide
NAT on a virtual network.  This enables some of the functionality one
can get with "user" network devices in QEMU.  I have a few use cases and
particularly want to be able to ssh into guests without requiring any
network configuration on the host.  This will be handy when using bhyve
to run CI jobs for FreeBSD.

The patch along with a more detailed description is here:
https://reviews.freebsd.org/D42510

Feedback and testing would be welcome.  The main issue currently is that
I'd like to be able to specify the same configuration parameter
("hostfwd") more than once, but the current config scheme appears to
make that difficult.  If anyone's interested in having guestfwd support
for some purpose, please say so.