Add RemainAfterExit equivalent for oneshot tasks

[troglobit]

The systemd RemainAfterExit is a clever construct along with oneshot tasks. In Finit terms such a run/task would still have the 'running' condition meaning other services could depend on it and post:script would still run when a user explicitly stops it, or the system goes down.

This issue should probably be done after #148

---

For a background, and more information, see https://github.com/orgs/finit-project/discussions/456