Fix advertised dispsize for libpq's sslmode connection parameter.

"8" was correct back when "disable" was the longest allowed value, but
since "verify-full" was added, it should be "12".  Given the lack of
complaints, I wouldn't be surprised if nobody is actually using these
values ... but still, if they're in the API, they should be right.

Noticed while pursuing a different problem.  It's been wrong for quite
a long time, so back-patch to all supported branches.

src/interfaces/libpq/fe-connect.c

@@ -181,7 +181,7 @@ static const PQconninfoOption PQconninfoOptions[] = {
 	 * to exclude them since none of them are mandatory.
 	 */
 	{"sslmode", "PGSSLMODE", DefaultSSLMode, NULL,
-	"SSL-Mode", "", 8},			/* sizeof("disable") == 8 */
+	"SSL-Mode", "", 12},			/* sizeof("verify-full") == 12 */
 
 	{"sslcert", "PGSSLCERT", NULL, NULL,
 	"SSL-Client-Cert", "", 64},