swanctl: Append /ESN to proposal for a CHILD_SA using Extended Sequence Numbers

We previously printed just the value for the "esn" keyword, which is "1", and
not helpful as such.

Fixes #904.

src/swanctl/commands/list_sas.c

@@ -122,7 +122,7 @@ CALLBACK(child_sas, int,
 		}
 		if (child->get(child, "esn"))
 		{
-			printf("/%s", child->get(child, "esn"));
+			printf("/ESN");
 		}
 		printf("\n");