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/check_config.h b/include/mbedtls/check_config.h
index cb707e9..2280cab 100644
--- a/include/mbedtls/check_config.h
+++ b/include/mbedtls/check_config.h
@@ -421,6 +421,11 @@
 #error "MBEDTLS_SSL_DTLS_HELLO_VERIFY  defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE) && \
+    ( !defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) || !defined(MBEDTLS_SSL_SRV_C) )
+#error "MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE  defined, but not all prerequisites"
+#endif
+
 #if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY) &&                              \
     ( !defined(MBEDTLS_SSL_TLS_C) || !defined(MBEDTLS_SSL_PROTO_DTLS) )
 #error "MBEDTLS_SSL_DTLS_ANTI_REPLAY  defined, but not all prerequisites"