Make varnishadm able to find args automatically

varnishadm already supported -n for getting the connection arguments.
However, unlike the other tools, you needed to pass an -n argument, it
did not default to anything.  It now uses the compiled-in default.

Fixes: #875

bin/varnishadm/varnishadm.c

@@ -355,7 +355,9 @@ main(int argc, char * const *argv)
 		if (sock < 0)
 			exit(2);
 	} else if (T_arg == NULL) {
-		usage();
+		sock = n_arg_sock("");
+		if (sock < 0)
+			exit(2);
 	} else {
 		assert(T_arg != NULL);
 		sock = cli_sock(T_arg, S_arg);