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

1 files changed, 57 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_TRAILERDATA.3 b/docs/libcurl/opts/CURLOPT_TRAILERDATA.3
new file mode 100644
index 000000000..32fadd4ac
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_TRAILERDATA.3
@@ -0,0 +1,57 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_TRAILERDATA 3 "14 Aug 2018" "libcurl 7.64.0" "curl_easy_setopt options"
+
+.SH NAME:
+CURLOPT_TRAILERDATA \- Custom pointer passed to the trailing headers callback
+		
+.SH SYNOPSIS:
+#include <curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TRAILERDATA, void *userdata);
+
+.SH DESCRIPTION:
+Data pointer to be passed to the HTTP trailer callback function.
+
+.SH DEFAULT:
+NULL
+
+.SH PROTOCOLS:
+HTTP
+
+.SH EXAMPLE:
+.nf
+/* Assuming we have a CURL handle in the hndl variable. */
+
+struct MyData data;
+
+curl_easy_setopt(hndl, CURLOPT_TRAILERDATA, &data);
+
+.fi
+
+A more complete example can be found in examples/http_trailers.html
+.SH AVAILABILITY:
+This option was added in curl 7.64.0 and is present if HTTP support is enabled
+
+.SH "SEE ALSO"
+.BR CURLOPT_TRAILERFUNCTION "(3), "