Portmap: append, rather than prepend, entry rules

This means that portmapped connections can be more easily controlled /
firewalled.

plugins/meta/portmap/portmap_test.go

@@ -323,6 +323,7 @@ var _ = Describe("portmapping configuration", func() {
 						"--mark", "0x20/0x20",
 						"-j", "MASQUERADE",
 					}},
+					prependEntry: true,
 				}))
 			})
 		})