Linux: Systemd unit improvements

[Thaodan]

Several improvements to the systemd units like increased security, simplicity, user service and system service template unit.

[mguessan]

That's a lot of changes, need to investigate and learn more recent stuff around systemd

[Thaodan]

Can you check again?

[coderkun]

Why does the user service use the template parameter? I will always be executed under the user who starts the service (that is what systemctl --user is for.

[Thaodan]

For example multiple instances of davmail per account. E.g. on work another for personal use.

[coderkun]

Fair enough 👍