MFC r344192 (by sef): Add support for a virtual hostname to nfsd

Specifically, this allows (via "-V vhostname") telling nfsd what principal
to use, instead of the hostname. This is used at iXsystems for fail-over in
HA systems.

Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D19191