sessionmonitor-systemd: prepare for D-Bus "user bus" model

In the D-Bus "user bus" model, all sessions of a user share the same
D-Bus instance, a polkit requesting process might live outside the
login session which registered the user's polkit agent.

In case a polkit requesting process is not part of the user's login
session, we ask systemd-logind for the the user's "display" session
instead.

https://bugs.freedesktop.org/show_bug.cgi?id=78905