Add (optional) pam_elogind.so support to the greeter.

Please consider merging this to close #932047.

The greeter needs to register its session with whichever logind implementation
is available so that reboot/suspend/halt commands are available to the user.

This patch adds optional pam_elogind.so support to /etc/pam.d/lightdm-greeter.

Whilst this approach is not future proof against other logind implementations
that may appear, it seems the least invasive solution.

As an alternative, it would also be possible to fix this by replacing both the
pam_systemd.so and pam_elogind.so optional lines with '@include common-session'.

I have tested both approaches with success.

Thanks

Mark