With-defaults CLI support

Extended cli_auto_show() with with-defaults argument, also changing its signature
Example: Added with-defaults argument to clispec
C-API: Added with-defaults argument to clicon_rpc_get_config
Replaced with-defaults prefix/namespace with constants

example/main/example_cli.c

@@ -71,7 +71,7 @@ mycallback(clicon_handle h, cvec *cvv, cvec *argv)
     /* Show eth0 interfaces config using XPATH */
     if (clicon_rpc_get_config(h, NULL, "running",
 			      "/interfaces/interface[name='eth0']",
-			      nsc,
+			      nsc, NULL, 
 			      &xret) < 0)
 	goto done;
     if (clixon_xml2file(stdout, xret, 0, 1, cligen_output, 0, 1) < 0)