units: let systemd --user manage its own memory pressure handling

units: let systemd --user manage its own memory pressure handling

Let's make things systematic: the per-user and the per-system manager
should manage their own memory pressure, as they are, well, managers of
things.

This is particularly relevant and the per-user service manager should
watch its own "init.scope" subcgroup, instead of the main service unit
cgroup, and hence $MEMORY_PRESSURE_WATCH as set by the per-system
service manager would simply be wrong.