introduce {fsck,mount}_vnd tools to be able to describe vnd images

in /etc/fstab instead of using some weird homegrown scripts.
No support for boot time mounting yet, so "noauto" is still needed.
original idea from david@
help and discussion todd@ bluhm@ beck@, manpage help jmc@
ok simon@ tedu@ bluhm@ todd@, "looks good" thib@

src/etc/rc

@@ -1,4 +1,4 @@
-#	$OpenBSD: rc,v 1.302 2007/03/14 20:47:17 ckuethe Exp $
+#	$OpenBSD: rc,v 1.303 2007/04/14 11:54:00 grunk Exp $
 
 # System startup script run by init on autoboot
 # or after single-user.
@@ -198,7 +198,7 @@ fi
 trap "echo 'Boot interrupted.'; exit 1" 3
 
 umount -a >/dev/null 2>&1
-mount -a -t nonfs
+mount -a -t nonfs,vnd
 mount -uw /		# root on nfs requires this, others aren't hurt
 rm -f /fastboot		# XXX (root now writeable)