ssh: add the ability to enable compression (for SCP/SFTP)

The required low-level logic was already available as part of
`libssh2` (via `LIBSSH2_FLAG_COMPRESS` `libssh2_session_flag()`[1]
option.)

This patch adds the new `libcurl` option `CURLOPT_SSH_COMPRESSION`
(boolean) and the new `curl` command-line option `--compressed-ssh`
to request this `libssh2` feature. To have compression enabled, it
is required that the SSH server supports a (zlib) compatible
compression method and that `libssh2` was built with `zlib` support
enabled.

[1] https://www.libssh2.org/libssh2_session_flag.html

Ref: https://github.com/curl/curl/issues/1732
Closes https://github.com/curl/curl/pull/1735

1 files changed, 58 insertions, 0 deletions

diff --git a/docs/libcurl/opts/CURLOPT_SSH_COMPRESSION.3 b/docs/libcurl/opts/CURLOPT_SSH_COMPRESSION.3
new file mode 100644
index 000000000..193320512
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_SSH_COMPRESSION.3
@@ -0,0 +1,58 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 1998 - 2017, 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_SSH_COMPRESSION 3 "05 Aug 2017" "libcurl 7.56.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_SSH_COMPRESSION \- enables automatic decompression of HTTP downloads
+.SH SYNOPSIS
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSH_COMPRESSION, long enable);
+.SH DESCRIPTION
+Pass a long as parameter to enable or disable.
+
+Enables built-in SSH compression.
+This is a request, not an order; the server may or may not do it.
+.SH DEFAULT
+
+.SH PROTOCOLS
+All SSH based protocols: SCP, SFTP
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com");
+
+  /* enable built-in compression */
+  curl_easy_setopt(curl, CURLOPT_SSH_COMPRESSION, 1L);
+
+  /* Perform the request */
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in 7.56.0
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, CURLE_UNKNOWN_OPTION if not, or
+CURLE_OUT_OF_MEMORY if there was insufficient heap space.
+.SH "SEE ALSO"
+.BR CURLOPT_ACCEPT_ENCODING "(3), " CURLOPT_TRANSFER_ENCODING "(3), "