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

lib/ssh.c

@@ -2942,6 +2942,13 @@ static CURLcode ssh_connect(struct connectdata *conn, bool *done)
     return CURLE_FAILED_INIT;
   }
 
+  if(data->set.ssh_compression) {
+#if LIBSSH2_VERSION_NUM >= 0x010208
+    if(libssh2_session_flag(ssh->ssh_session, LIBSSH2_FLAG_COMPRESS, 1) < 0)
+#endif
+      infof(data, "Failed to enable compression for ssh session\n");
+  }
+
 #ifdef HAVE_LIBSSH2_KNOWNHOST_API
   if(data->set.str[STRING_SSH_KNOWNHOSTS]) {
     int rc;

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;

lib/urldata.h

@@ -1742,6 +1742,7 @@ struct UserDefined {
 
   curl_sshkeycallback ssh_keyfunc; /* key matching callback */
   void *ssh_keyfunc_userp;         /* custom pointer to callback */
+  bool ssh_compression;            /* enable SSH compression */
 
 /* Here follows boolean settings that define how to behave during
    this session. They are STATIC, set by libcurl users or at least initially