sftp: Allow compression if necessary

Specifying only "none" as compression method causes the exchange to fail if
the server doesn't support "none".

Use the same options as OpenSSH does by default. The Compression=yes/no option
in the config file overrides them if specified.

Setting the options explicitly can be dropped in the future after libssh
adjusts its built-in defaults.

(cherry picked from commit c5a9722d)