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)
4c109df9b59e55699bd42023cf4007ee359793e9

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
Do not mask virtual file systems when using systemctl (#708378)

This enhancement removes the virtual file systems from the list of file
systems (shown below) to be masked.

The following output was captured using Fedora 19:

$ systemctl list-units --full --all -t mount
UNIT                          LOAD   ACTIVE   SUB     DESCRIPTION
-.mount                       loaded active   mounted /
boot.mount                    loaded active   mounted /boot
dev-hugepages.mount           loaded active   mounted Huge Pages File System
dev-mqueue.mount              loaded active   mounted POSIX Message Queue File System
proc-sys-fs-binfmt_misc.mount loaded inactive dead    Arbitrary Executable File Formats File System
run-user-1000-gvfs.mount      loaded active   mounted /run/user/1000/gvfs
sys-fs-fuse-connections.mount loaded active   mounted FUSE Control File System
sys-kernel-config.mount       loaded active   mounted Configuration File System
sys-kernel-debug.mount        loaded active   mounted Debug File System
tmp.mount                     loaded active   mounted Temporary Directory

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

10 loaded units listed.
To show all installed unit files use 'systemctl list-unit-files'.

Bug #708378 - Advertised new feature: Use systemctl runtime mask to
              prevent automounting (#701676) doesn't work
Update install instructions for some GNU/Linux distributions