bhyve: Add a slirp network backend

This enables a subset of the functionality provided by QEMU's user
networking implementation.  In particular, it uses net/libslirp, the
same library as QEMU.

libslirp is permissively licensed but has some dependencies which make
it impractical to bring into the base system (glib in particular).  I
thus opted to make bhyve dlopen the libslirp.so, which can be installed
via pkg.  The library header is imported into bhyve.

The slirp backend takes a "hostfwd" which is identical to QEMU's
hostfwd.  When configured, bhyve opens a host socket and listens for
connections, which get forwarded to the guest.  For instance,
"hostfwd=tcp::1234-:22" allows one to ssh into the guest by ssh'ing to
port 1234 on the host, e.g., via 127.0.0.1.  I didn't try to hook up
guestfwd support since I don't personally have a use-case for it yet,
and I think it won't interact nicely with the capsicum sandbox.

Reviewed by:	jhb
Tested by:	rew
MFC after:	1 month
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D42510

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