Restore the "plumb" compatibility command, which was lost

due to the recent change by sam@ to clone operations in ifconfig(8).

sbin/ifconfig/ifclone.c

@@ -145,6 +145,7 @@ DECL_CMD_FUNC(clone_destroy, arg, d)
 static struct cmd clone_cmds[] = {
 	DEF_CMD("create",	0,	clone_create),
 	DEF_CMD("destroy",	0,	clone_destroy),
+	DEF_CMD("plumb",	0,	clone_create),
 	DEF_CMD("unplumb",	0,	clone_destroy),
 };