Add support for the new mountd -R option.

r376026 added a new "-R" option to mountd, which tells it to
not support the Mount protocol (not used by NFSv4) and not
register with rpcbind.
Rpcbind is considered a security issue by some sites now.

This patch adds a new yes/no variable called nfsv4_server_only.
When that is set, make vfs.nfsd.server_min_vers=4 and set "=R"
for mountd.
Setting vfs.nfsd.server_min_vers=4 tells nfsd to not register with rpcbind.
While here, add a check for "load_kld nfsd" failing to nfsd.

Reviewed by: 0mp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D26938