PUP-1235 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.
@peiriannydd how about adding a feature to the provider and making this conditional on providers that support the feature, so you don't have to do `respond_to`?

<pre>
newvalue(:mask, :event => :service_disabled, required_features => :maskable)
</pre>