lightdm doesn't let pam_mkhomedir execute

Affects:
14.04 LTS
lightdm 1.10.0-0ubuntu3

When graphically logging in with a new user without a home directory, lightdm/xdg/freedesktop creates a home directory based on settings in /etc/xdg before PAM session configs are processed. This makes pam_mkhomedir completely useless for graphical logins of first-time users. It also creates inconsistencies between graphical and console logins as the console still processes mkhomedir. Thus, two new users would have differing home directories if one initially logged in graphically vs intially logged in via a console.

This bug does not exist in 12.04 LTS.

How to reproduce:
1. Create a new user, ensuring the user has no home directory.
2. Configure pam with pam_mkhomedir to create a home directory by copying files from some skeleton directory.
3. Login via lightdm with the new user.
4. Verify that none of the files from the skeleton are actually in the new user's home directory.

What is expected to happen is that pam_mkhomedir is always processed before any other home directory files are created, enabling administrators to dictate the home directory contents for new users using the traditional method of pam_mkhomedir + /etc/skel.