Bring in NFS support for virtual hostnames
Specifically, this allows (via "-V vhostname") telling nfsd what principal to use, instead of the hostname. This is used for HA fail-over.
FreeBSD review https://reviews.freebsd.org/D19191
The proposed change there is slightly different from what we've got right now; assuming it gets in, I'll merge in the command-line option to our code so it'll support both, until we get it working with the command-line option.