act-user-manager: Find the user's graphical session instead of assuming we are in it
Submitted by Iain Lane
Assigned to Matthias Clasen
See https://wiki.gnome.org/Initiatives/SystemdUser for more details - basically if programs are started by systemd --user then you can't ask "what session am I in?" because the answer is "none". Instead it's "what is the display session?".
This patch should be safe to merge immediately & it is a prerequisite for the systemd --user work.