- Set a nicer unit description.
- Use `network.target` instead of `network-online.target`.
As per https://www.freedesktop.org/software/systemd/man/latest/systemd.special.html#network-online.target,
mycelium doesn't strictly require a configured network connection
(whatever that means).
`network.target` should be sufficient, we probably need the IP stack
and a loopback interface, but everything else should be OK to change
later on.
- Use `StateDirectory=mycelium` and -k `%S/mycelium-key.bin`.
Key material is state, not config, especially if it's autocreated
on first startup. Combine with `StateDirectoryMode=0700`, to ensure
the directory can't be accessed by other users.
