systemctl: usable status command for special units (#4072)

systemctl: usable status command for special units (#4072)

Prior to this commit, users could be given an unusable command to run if
they attempted to stop or start special services.  For example:

    $ systemctl stop -- -.mount
    Failed to stop -.mount: Operation refused, unit -.mount may be \
        requested by dependency only.
    See system logs and 'systemctl status -.mount' for details.

    $ systemctl status -.mount
    systemctl: invalid option -- '.'

This adds a '--' to the example command in these situations.