Handle reassembly of handshake messages

Works only with GnuTLS for now, OpenSSL packs other records in the same
datagram after the last fragmented one, which we don't handle yet.

Also, ssl-opt.sh fails the tests with valgrind for now: we're so slow with
valgrind that gnutls-serv retransmits some messages, and we don't handle
duplicated messages yet.

include/polarssl/ssl.h

@@ -625,6 +625,7 @@ struct _ssl_handshake_params
                                               Srv: unused                    */
     unsigned char verify_cookie_len;    /*!<  Cli: cookie length
                                               Srv: flag for sending a cookie */
+    unsigned char *hs_msg;              /*!<  Reassembled handshake message  */
 #endif
 
     /*