MFC: r307890
mountd(8) was erroneously setting the sysctl for the old NFS server
when the new/default NFS server was running, for the "-n" option.

This patch fixes the problem for stable/10 and stable/9.
Since the new NFS server uses vfs.nfsd.nfs_privport == 0 by default,
there wouldn't have been many users affected by the code not setting
it to 0 when the "-n" option was specified.