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

src/tool_help.c

@@ -68,6 +68,8 @@ static const struct helptxt helptext[] = {
    "SSL ciphers to use"},
   {"    --compressed",
    "Request compressed response"},
+  {"    --compressed-ssh",
+   "Enable SSH compression"},
   {"-K, --config <file>",
    "Read config from a file"},
   {"    --connect-timeout <seconds>",