Set the default nfsd flags to "-tun 4". Matches the comment

in rc.conf and the behaviour of the backwards compatibility
code in rc.subr for nfs_server=YES.
ok ajacoutot@

src/etc/rc.d/nfsd

@@ -1,8 +1,9 @@
 #!/bin/sh
 #
-# $OpenBSD: nfsd,v 1.5 2012/08/14 07:43:57 ajacoutot Exp $
+# $OpenBSD: nfsd,v 1.6 2014/08/22 08:10:38 jsg Exp $
 
 daemon="/sbin/nfsd"
+daemon_flags="-tun 4"
 
 . /etc/rc.d/rc.subr