systemd-user: Drop RefuseManualStart=true.

* doc/examples/systemd-user/*.service: drop RefuseManualStart=true

These user services can be safely started manually as long as at least
their primary sockets are available. They'll just start with nothing
to do, which should be fine.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>