rpm: restart user services at the end of the transaction

This closes an important gap: so far we would reexecute the system manager and
restart system services that were configured to do so, but we wouldn't do the
same for user managers or user services.

The scheme used for user managers is very similar to the system one, except
that there can be multiple user managers running, so we query the system
manager to get a list of them, and then tell each one to do the equivalent
operations: daemon-reload, disable --now, set-property Markers=+needs-restart,
reload-or-restart --marked.

The total time that can be spend on this is bounded: we execute the commands in
parallel over user managers and units, and additionally set SYSTEMD_BUS_TIMEOUT
to a lower value (15 s by default). User managers should not have too many
units running, and they should be able to do all those operations very
quickly (<< 1s). The final restart operation may take longer, but it's done
asynchronously, so we only wait for the queuing to happen.

The advantage of doing this synchronously is that we can wait for each step to
happen, and for example daemon-reloads can finish before we execute the service
restarts, etc. We can also order various steps wrt. to the phases in the rpm
transaction.

When this was initially proposed, we discussed a more relaxed scheme with bus
property notifications. Such an approach would be more complex because a bunch
of infrastructure would have to be added to system manager to propagate
appropriate notifications to the user managers, and then the user managers
would have to wait for them. Instead, now there is no new code in the managers,
all new functionality is contained in src/rpm/. The ability to call 'systemctl
--user user@' makes this approach very easy. Also, it would be very hard to
order the user manager steps and the rpm transaction steps.

Note: 'systemctl --user disable' is only called for a user managers that are
running. I don't see a nice way around this, and it shouldn't matter too much:
we'll just leave a dangling symlink in the case where the user enabled the
service manually.

A follow-up for https://bugzilla.redhat.com/show_bug.cgi?id=1792468 and
fa97d2fc.