systemd: Update from v211 to nixos-v212
systemd: Add an assertion to guarantee oneshot units do not have restart set

This prevents insidious errors once systemd begins handling the unit. If
the unit is loaded at boot, any errors of this nature are logged to the
console before the journal service is running. This makes it very hard
to diagnose the issue. Therefore, this assertion helps guarantee the
mistake is not made.
upstart: Oneshot rules should always have Restart=no