Change systemctl to also mask /etc/fstab mount entries (#708378)

A mistake was made in the following commit:

Use systemctl runtime mask to prevent automounting (#701676)
4c109df9

The intention was to use 'systemctl list-units' rather than
'systemctl list-unit-files' so that auto-generated mount files would
also be masked and hence prevented from auto-mounting.

Now 'systemctl list-units' is used.

Bug #708378 - Advertised new feature: Use systemctl runtime mask to
              prevent automounting (#701676) doesn't work