properly specify location of sftp-server binary

Currently there is a symlink at /usr/lib/sftp-server to /usr/lib/openssh/sftp-server in the image. That seems to give trouble with packages like the rssh restricted shell, which will not follow a symlink to the sftp server binary for security reasons. So currently it is not possible to use this image as a basis for scp/sftp only access over ssh.
Merge pull request #38 from slawo-ch/patch-1

properly specify location of sftp-server binary