Implement Maximum Fragment Length TLS extension.

Based on patch from Tomasz Moń:
https://groups.google.com/forum/#!topic/mailing.openssl.dev/fQxXvCg1uQY

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/1008)

util/perl/TLSProxy/Message.pm

@@ -63,6 +63,7 @@ my %message_type = (
 
 use constant {
     EXT_SERVER_NAME => 0,
+    EXT_MAX_FRAGMENT_LENGTH => 1,
     EXT_STATUS_REQUEST => 5,
     EXT_SUPPORTED_GROUPS => 10,
     EXT_EC_POINT_FORMATS => 11,