Register delegate to get callbacks about the CURL transfer

Property definitions

curl $ NativeCurl :: register_callback_header
	# Register `delegate` to get callbacks about the CURL transfer
	fun register_callback_header(delegate: NativeCurlCallbacks): CURLCode
	import CString.native_callback_header `{
		CURLcode e;
		NativeCurlCallbacks_incr_ref(delegate); // FIXME deallocated these when download completes?

		e = curl_easy_setopt(self, CURLOPT_HEADERFUNCTION, (curl_write_callback)&CString_native_callback_header);
		if(e != CURLE_OK) return e;

		e = curl_easy_setopt(self, CURLOPT_WRITEHEADER, delegate);
		return e;
	`}
lib/curl/native_curl.nit:204,2--215,3