This changes log2ram dependencies to start as early as possible, before other
services that may require access to /var/log, including rsyslog and
systemd-journald.

Additional dependencies on shutdown.target are required to ensure that
the service will be stopped at shutdown/reboot when DefaultDependencies
is set to "no" [1].

RequiresMountsFor declares dependency on /var mount when it is a mount point.

IgnoreOnIsolate prevents stopping the service when isolating another unit.

[1]: https://www.freedesktop.org/software/systemd/man/systemd.service.html

From @datenklause
With Type=oneshot, systemd waits until the start action has completed
before it considers the unit to be active, so dependencies start only
after the start action has succeeded. With Type=simple, dependencies
will start immediately after the start action has been dispatched.

This tweak is required to make it run under Ubuntu 16.04