Rewrite error message related to sslmode in libpq

The same error message will be used for a different option, to be
introduced in a separate patch.  Reshaping the error message as done
here saves in translation.

Extracted from a larger patch by the same author.

Author: Jacob Champion
Discussion: https://postgr.es/m/9e5a8ccddb8355ea9fa4b75a1e3a9edc88a70cd3.camel@vmware.com

Author: michaelpq

src/interfaces/libpq/fe-connect.c

@@ -1457,8 +1457,8 @@ connectOptions2(PGconn *conn)
 			case 'r':			/* "require" */
 			case 'v':			/* "verify-ca" or "verify-full" */
 				conn->status = CONNECTION_BAD;
-				libpq_append_conn_error(conn, "sslmode value \"%s\" invalid when SSL support is not compiled in",
-										conn->sslmode);
+				libpq_append_conn_error(conn, "%s value \"%s\" invalid when SSL support is not compiled in",
+										"sslmode", conn->sslmode);
 				return false;
 		}
 #endif