Fix race in io.systemd.Machine.Open() test case (#36410)

This started out as a simple attempt to fix a race in an existing test
case for io.systemd.Machine.Open(). To address it nicely I added some
machinery to varlinkctl and systemd-notify though, and because of that I
refacting reception of sd_notify() messages in various places of our
codebase. So it became much much bigger.

This ports all receivers of sd_notify() messages over to a new common
implementation, except for one: the one in PID1. It's more powerful than
the others, since it accepts fds too. I think we should generalize
notify_recv() to cover that too, it's not that much more work, but for
now I didn't want to add even more refactorings on top, and this can
easily happen later separately, hence I left it out for now.