Start adding srv support for hello verify request

Dummy fixed content for now.

Also, seems to be a race condition in the way the socket is closed and
reopened, leading to a few "random" failures in compat.sh. A fix is planned
for later.

include/polarssl/ssl.h

@@ -146,6 +146,7 @@
 #define POLARSSL_ERR_SSL_INTERNAL_ERROR                    -0x6C00  /**< Internal error (eg, unexpected failure in lower-level module) */
 #define POLARSSL_ERR_SSL_COUNTER_WRAPPING                  -0x6B80  /**< A counter would wrap (eg, too many messages exchanged). */
 #define POLARSSL_ERR_SSL_WAITING_SERVER_HELLO_RENEGO       -0x6B00  /**< Unexpected message at ServerHello in renegotiation. */
+#define POLARSSL_ERR_SSL_HELLO_VERIFY_REQUIRED             -0x6A80  /**< DTLS client must retry for hello verification */
 
 /*
  * Various constants