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

tests/data/test642

@@ -0,0 +1,42 @@
+<testcase>
+<info>
+<keywords>
+SFTP
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data>
+Test data
+for ssh test
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+sftp
+</server>
+ <name>
+SFTP retrieval
+ </name>
+ <command>
+--key curl_client_key --pubkey curl_client_key.pub -u %USER: --compressed-ssh sftp://%HOSTIP:%SSHPORT%PWD/log/file642.txt --insecure
+</command>
+<file name="log/file642.txt">
+Test data
+for ssh test
+</file>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<valgrind>
+disable
+</valgrind>
+</verify>
+</testcase>