oom: Add support for user unit ManagedOOM property updates

oom: Add support for user unit ManagedOOM property updates

Compared to PID1 where systemd-oomd has to be the client to PID1
because PID1 is a more privileged process than systemd-oomd, systemd-oomd
is the more privileged process compared to a user manager so we have
user managers be the client whereas systemd-oomd is now the server.

The same varlink protocol is used between user managers and systemd-oomd
to deliver ManagedOOM property updates. systemd-oomd now sets up a varlink
server that user managers connect to to send ManagedOOM property updates.

We also add extra validation to make sure that non-root senders don't
send updates for cgroups they don't own.

The integration test was extended to repeat the chill/bloat test using
a user manager instead of PID1.