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, 3 insertions, 0 deletions

diff --git a/lib/url.c b/lib/url.c
index 71d4d8bb7..32623abe5 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -2936,6 +2936,9 @@ CURLcode Curl_setopt(struct Curl_easy *data, CURLoption option,
   case CURLOPT_SUPPRESS_CONNECT_HEADERS:
     data->set.suppress_connect_headers = (0 != va_arg(param, long))?TRUE:FALSE;
     break;
+  case CURLOPT_SSH_COMPRESSION:
+    data->set.ssh_compression = (0 != va_arg(param, long))?TRUE:FALSE;
+    break;
   default:
     /* unknown tag and its companion, just ignore: */
     result = CURLE_UNKNOWN_OPTION;