Modify /etc/rc.d/nfsd so it doesn't force a startup of nfsuserd for NFSv4.

Given that RFC7530 allows uid/gids to be placed in owner/owner_group
strings directly, many NFSv4 environments don't need the nfsuserd.
This small patch modified /etc/rc.d/nfsd so that it does not force
startup of the nfsuserd daemon unless nfs_server_managegids is enabled.
This implies that nfsuserd_enable="YES" must be added to /etc/rc.conf
for NFSv4 server environments that use Kerberos mounts or clients that
do not support the uid/gid in string capability.
Since this could be considered a POLA violation, it will not be MFC'd.

Discussed on: freebsd-current