feat(systemd): Add support for unmasking & masking units (#88)

* feat(systemd): Add support for masking services

* Use unit term in README

* Fix README grammar

* Note about unmasking

* Fix typo in README

* Add unmask & more info about systemctl commands

Address:

https://github.com/ublue-os/bling/pull/88#discussion_r1436877565

https://github.com/ublue-os/bling/pull/88#discussion_r1436917124

* docs: clarify purpose of unmasking

* docs: Fix systemd module README typo

---------

Co-authored-by: xyny <60004820+xynydev@users.noreply.github.com>

modules/systemd/README.md

@@ -1,6 +1,6 @@
 # `systemd` Module for Startingpoint
 
-The `systemd` module streamlines the management of systemd units during image building. Units are divided into `system` and `user` categories, with `system` units managed directly using `systemctl` and `user` units using `systemctl --user`. You can specify which units to enable or disable under each category.
+The `systemd` module streamlines the management of systemd units during image building. Units are divided into `system` and `user` categories, with `system` units managed directly using `systemctl` and `user` units using `systemctl --user`. You can specify which units to enable/disable or unmask/mask under each category.
 
 ## Example Configuration
 
@@ -11,22 +11,34 @@ system:
     - example.service
   disabled:
     - example.target
+  unmasked:
+    - example.service    
+  masked:
+    - example.service
 user:
   enabled:
     - example.timer
   disabled:
     - example.service
+  unmasked:
+    - example.service    
+  masked:
+    - example.service
 ```
 
 In this example:
 
 ### System Units
 - `example.service`: Enabled (runs on system boot)
-- `example.target`: Disabled (does not run on system boot)
+- `example.target`: Disabled (does not run on system boot, unless other unit strictly requires it)
+- `example.service`: Unmasked (runs on system boot, even if previously masked)
+- `example.service`: Masked (does not run on system boot, under any circumstances)
 
 ### User Units
 - `example.timer`: Enabled (runs for the user)
-- `example.service`: Disabled (does not run for the user)
+- `example.service`: Disabled (does not run for the user, unless other unit strictly requires it)
+- `example.service`: Unmasked (runs for the user, even if previously masked)
+- `example.service`: Masked (does not run for the user, under any circumstances)
 
 This configuration achieves the same results as the following commands:
 
@@ -34,8 +46,18 @@ This configuration achieves the same results as the following commands:
 # System Units
 systemctl enable example.service
 systemctl disable example.target
+systemctl unmask example.service 
+systemctl mask example.service 
 
 # User Units
-systemctl --user enable example.timer
-systemctl --user disable example.service
+systemctl --global enable example.timer
+systemctl --global disable example.service
+systemctl --global unmask example.service
+systemctl --global mask example.service
 ```
+
+For more information about these systemctl commands, please visit: 
+https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#enable%20UNIT%E2%80%A6
+https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#disable%20UNIT%E2%80%A6
+https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#unmask%20UNIT%E2%80%A6
+https://www.freedesktop.org/software/systemd/man/latest/systemctl.html#mask%20UNIT%E2%80%A6