Comment 37 for bug 1359439

This is actually in systemd, not systemd-shim.
I patched systemd-shim to ignore unknown unit warning, so then I get a warning from systemd-logind: systemd-logind[1414]: Failed to start user service: Message did not receive a reply (timeout by message bus)
So it's being started by the login manager once pam-systemd registers the session.

pam_systemd manpage: A new systemd scope unit is created for the session. If this is the first concurrent session of the user, an implicit slice below user.slice is automatically created and the scope placed
           into it. An instance of the system service user@.service, which runs the systemd user manager instance, is started.

I'm guessing user@.service is generated at runtime by systemd (which would require systemd is init, not upstart).

Suggestions for fixing this:
1. Move /etc/pam.d/systemd-user to the systemd-sysv package (assuming none of systemd-logind's features are required)
2. Patch systemd-logind to not start the user service if the system wasn't booted with systemd

I'll take a look at patching systemd-logind.