Activation script reintroduction (support non-systemd systems)

[jordanisaacs]

Add option to use script instead of systemd for startup secret decryption.

Waiting on v0.1 stabilization of option set and writing checks/cleanup.

[Gerschtli]

This would also add support for other linux distros where no systemd is available, e.g. Windows WSL.

[jordanisaacs]

Currently available on main branch, will be released with v0.1