lib: send eos flag

Adds a `bool eos` flag to send methods to indicate that the data is the
last chunk the invovled transfer wants to send to the server.

This will help protocol filters like HTTP/2 and 3 to forward the
stream's EOF flag and also allow to EAGAIN such calls when buffers are
not yet fully flushed.

Closes #14220

lib/cf-haproxy.c

@@ -132,7 +132,7 @@ static CURLcode cf_haproxy_connect(struct Curl_cfilter *cf,
       size_t written;
       result = Curl_conn_send(data, cf->sockindex,
                               Curl_dyn_ptr(&ctx->data_out),
-                              len, &written);
+                              len, FALSE, &written);
       if(result == CURLE_AGAIN) {
         result = CURLE_OK;
         written = 0;