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 head and stable/11. For stable/10 the
patch will need to be modified when MFC'd, since the stable/10 mountd.c
handles both old and new NFS servers.
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.

PR: 213450
Submitted by: rs@bytecamp.net
MFC after: 2 weeks