Export autologin-session to greeter

This is a complementary bug report to https://bugs.launchpad.net/lightdm-gtk-greeter/+bug/1707186, since the fix of this bug requires patching both, lightdm and lighthdm-gtk-greater.

Imported description from the other report:

When lightdm is configured with with 'autologin-user-timeout' > 0
any configured 'autologin-session' will be ignored and the autologin-user
will be logged in with the last used session instead (read from
/var/lib/lightdm/.cache/lightdm-gtk-greeter/state).

Example 1:

    autologin-user=foo
    autologin-user-timeout=0
    autologin-session=fluxbox

This works as intended, as the autologin is handled directly by 'lightdm'.

Example 2:

    autologin-user=foo
    autologin-user-timeout=10
    autologin-session=fluxbox

Now the autologin must be handled by the greeter, and this results
into starting the (last) selected session, which must not be the
same as the configured 'autologin-session'.

The fix is to explicitly set the session before doing the autologin.
But since the greeter hints doesn't give us any information about the
'autologin-session' a second patch to lightdm is also required).
However, since the attached patch to lightdm-gtk-greeter will just
keep the current behaviour if the second patch hasn't been applied
to lightdm it would be save to merge this anytime.