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.
On systemd 208, we don't need to execute and parse another command: 

```
grey# systemctl is-enabled ModemManager
disabled
grey# echo $?
1
grey# systemctl mask ModemManager      
ln -s '/dev/null' '/etc/systemd/system/ModemManager.service'
grey# systemctl is-enabled ModemManager
masked
grey# echo $?                          
1
grey# systemctl --version              
systemd 208
+PAM +LIBWRAP -AUDIT -SELINUX +IMA -SYSVINIT +LIBCRYPTSETUP -GCRYPT +ACL +XZ
```