sftp: Allow compression if necessary

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

I'm really wondering whether it makes sense still to not compress data. In the vast majority of cases, (de)compression is faster than the connection and zlib has minimal overhead for uncompressible data. (CC @sitter)

Downstream report (slightly different scenario, needs fix in libssh): https://bugzilla.opensuse.org/show_bug.cgi?id=1192731

Draft because not confirmed by the reporter yet. Still haven't heard back from the reporter, but it works as expected here.