Cloning via SFTP not working

On newer systems the scp command will use sftp internally which will cause operations to break because we currently do not have a working SFTP repository access layer.

A workaround is described in the README but this is inconvenient and also the SCP method is notably slower than the SFTP one.

Proposed solution

Implement a SftpSubsystem in the SshServer and extend the DarcsSshCommand accordingly. The implemented subsystem MUST respect the permissions for repositories like the already existing SCP implementation.

Hint: Check if this enables support for the currently disabled darcs transfer-mode command.