Replace reboot-guard

[Eeems]

Is your feature request related to a problem? Please describe.
Reboot guard requires pulling down python3, which greatly increases the size of an oxide install.

Describe the solution you'd like
Manually implement what reboot-guard does.

Describe alternatives you've considered
Compile reboot-guard with nuitka to avoid needing dependencies.

Additional context
Reboot-guard's functionality shouldn't be too difficult to manually implement as it just adds an override the poweroff, reboot, and halt targets with the following:

[Unit]
RefuseManualStart=yes