PUP-1253 Add systemd service masking support

This updated the systemd service type to add support for masking. If a
service is masked, it is deemed to also be disabled. If a service is
masked and changed to enabled, it will first be unmasked since the
standard 'systemctl enable' command does not properly unmask the command
first.
If `:false` going to be returned? Or is it going to be nil? I think there should be a line stating `:false` in this block.

Should it really be `:first` as opposed to `false` ?