Explicitly mount the NFS images ro, instead of relying on the server to

export it ro.  This saves thrashing the server if we accidentally try to
rm -rf a NFS mount (e.g. because a chroot cleanup failed).

Tools/portbuild/scripts/portbuild

@@ -11,7 +11,7 @@ mount_fs()
     if [ ${disconnected} = 1 ]; then
 	mount -t nullfs -r ${fs} ${mntpt}
     else
-	mount -o nfsv3,intr -r ${master}:${fs} ${mntpt}
+	mount_nfs -o ro -3 -i ${master}:${fs} ${mntpt}
     fi
 }