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/Makefile.inc

@@ -77,7 +77,7 @@ test599 test600 test601 test602 test603 test604 test605 test606 test607 \
 test608 test609 test610 test611 test612 test613 test614 test615 test616 \
 test617 test618 test619 test620 test621 test622 test623 test624 test625 \
 test626 test627 test628 test629 test630 test631 test632 test633 test634 \
-test635 test636 test637 test638 test639 test640 test641 \
+test635 test636 test637 test638 test639 test640 test641 test642 \
 \
 test700 test701 test702 test703 test704 test705 test706 test707 test708 \
 test709 test710 test711 test712 test713 test714 test715 \

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>