Support IPv6 in rump.  I'd have liked to introduce a netinet6
component, but due to ifdef happiness permeating the sources, it's
a compile decision for now, so netinet pulls in both inet and inet6.

One issue, one single issue: the loopback interface still needs to
be created for IPv6 to work.  I have patches to take care of it
automatically if the appropriate component (net) is present, but
they require a bit more testing before commit.

(pooka)