Stop wasting resources

Use a custom function that minimally parses the message an creates a reply
without the overhead of a full SSL context.

Also fix dependencies: needs DTLS_HELLO_VERIFY for the cookie types, and let's
also depend on SRV_C as is doesn't make sense on client.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 4956d04..381e82b 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1144,6 +1144,9 @@
  * new connection securely, as described in section 4.1.8 of RFC 6347. This
  * flag enables that support.
  *
+ * Requires: MBEDTLS_SSL_DTLS_HELLO_VERIFY
+ *           MBEDTLS_SSL_SRV_C
+ *
  * Comment this to disable support for clients reusing the source port.
  */
 #define MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE