http: Implement trailing headers for chunked transfers

This adds the CURLOPT_TRAILERDATA and CURLOPT_TRAILERFUNCTION
options that allow a callback based approach to sending trailing headers
with chunked transfers.

The test server (sws) was updated to take into account the detection of the
end of transfer in the case of trailing headers presence.

Test 1591 checks that trailing headers can be sent using libcurl.

Closes #3350

lib/http.c

@@ -1681,6 +1681,52 @@ enum proxy_use {
   HEADER_CONNECT  /* sending CONNECT to a proxy */
 };
 
+/* used to compile the provided trailers into one buffer
+   will return an error code if one of the headers is
+   not formatted correctly */
+CURLcode Curl_http_compile_trailers(struct curl_slist *trailers,
+                                    Curl_send_buffer *buffer,
+                                    struct Curl_easy *handle)
+{
+  char *ptr = NULL;
+  CURLcode result = CURLE_OK;
+  const char *endofline_native = NULL;
+  const char *endofline_network = NULL;
+
+  /* TODO: Maybe split Curl_add_custom_headers to make it reusable here */
+
+  if(
+#ifdef CURL_DO_LINEEND_CONV
+     (handle->set.prefer_ascii) ||
+#endif
+     (handle->set.crlf)) {
+    /* \n will become \r\n later on */
+    endofline_native  = "\n";
+    endofline_network = "\x0a";
+  }
+  else {
+    endofline_native  = "\r\n";
+    endofline_network = "\x0d\x0a";
+  }
+
+  while(trailers) {
+    /* only add correctly formatted trailers */
+    ptr = strchr(trailers->data, ':');
+    if(ptr && *(ptr + 1) == ' ') {
+      result = Curl_add_bufferf(&buffer, "%s%s", trailers->data,
+                                endofline_native);
+      if(result)
+        return result;
+    }
+    else
+      infof(handle, "Malformatted trailing header ! Skipping trailer.");
+    trailers = trailers->next;
+  }
+  result = Curl_add_buffer(&buffer, endofline_network,
+                           strlen(endofline_network));
+  return result;
+}
+
 CURLcode Curl_add_custom_headers(struct connectdata *conn,
                                  bool is_connect,
                                  Curl_send_buffer *req_buffer)