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.
I've never seen someone actually use the `:event` option for values, is this being used somewhere?