New extension callback features.

Support separate parse and add callback arguments.
Add new callback so an application can free extension data.
Change return value for send functions so < 0 is an error 0
omits extension and > 0 includes it. This is more consistent
with the behaviour of other functions in OpenSSL.

Modify parse_cb handling so <= 0 is an error.

Make SSL_CTX_set_custom_cli_ext and SSL_CTX_set_custom_cli_ext argument
order consistent.

NOTE: these changes WILL break existing code.

Remove (now inaccurate) in line documentation.
Reviewed-by: Emilia Käsper <emilia@openssl.org>

apps/s_client.c

@@ -1363,7 +1363,7 @@ bad:
 				{
 				SSL_CTX_set_custom_cli_ext(ctx,
 							   serverinfo_types[i],
-							   NULL, 
+							   NULL, NULL, NULL,
 							   serverinfo_cli_cb,
 							   NULL);
 				}