ssh_util: Fix specifying the SSH master for scp.

Both the scp and ssh programs either accept -S or directly using the
ControlPath ssh_config option, so let's only use the latter so we don't
interefere with scp, because when looking at scp(1):

  -S program
    Name of program to use for the encrypted connection. The program
    must understand ssh(1) options.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>