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_cfgable.h

@@ -247,6 +247,7 @@ struct OperationConfig {
                                      from user callbacks */
   curl_error synthetic_error;     /* if non-zero, it overrides any libcurl
                                      error */
+  bool ssh_compression;           /* enable/disable SSH compression */
   struct GlobalConfig *global;
   struct OperationConfig *prev;
   struct OperationConfig *next;   /* Always last in the struct */

src/tool_getparam.c

@@ -188,6 +188,7 @@ static const struct LongShort aliases[]= {
   {"$W", "abstract-unix-socket",     ARG_STRING},
   {"$X", "tls-max",                  ARG_STRING},
   {"$Y", "suppress-connect-headers", ARG_BOOL},
+  {"$Z", "compressed-ssh",           ARG_BOOL},
   {"0",   "http1.0",                 ARG_NONE},
   {"01",  "http1.1",                 ARG_NONE},
   {"02",  "http2",                   ARG_NONE},
@@ -1076,6 +1077,9 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
       case 'Y': /* --suppress-connect-headers */
         config->suppress_connect_headers = toggle;
         break;
+      case 'Z': /* --compressed-ssh */
+        config->ssh_compression = toggle;
+        break;
       }
       break;
     case '#': /* --progress-bar */

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>",

src/tool_operate.c

@@ -1091,6 +1091,10 @@ static CURLcode operate_do(struct GlobalConfig *global,
              to fail if we are not talking to who we think we should */
           my_setopt_str(curl, CURLOPT_SSH_HOST_PUBLIC_KEY_MD5,
                         config->hostpubmd5);
+
+          /* new in libcurl 7.56.0 */
+          if(config->ssh_compression)
+            my_setopt(curl, CURLOPT_SSH_COMPRESSION, 1L);
         }
 
         if(config->cacert)