Bug#932047: lightdm: greeter session support for elogind

[Sam Hartman <hartmans@debian.org>]

Hi.

If including common-session will work, I think that's a good improvement
for everyone.
It is closer to best practice, and it means that as PAM profiles are
added over time, they will work for lightdm as well.

Whether that works depends on the architecture of the greeter.
If the greeter has one process that does the initial authentication and
then forks off an entire different set of processes not descended from
the greeter that run the session, then including common-session might
not work so well.

I'm kind of confused though because it looks like  1.26.0-8's sources
already include common-session in data/pam/lightdm.