[WIP] Lightdm service

[L p R n d n]

Hello,

I'm trying to write a service for lightdm but I'm currently stuck but
maybe some of you might have a clue.
FYI, relevant patches can be found here:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=35305

The problem is that the service is unable to start a session. After
login, lightdm just black screen and restart.

Looking at logs, the only relevant problem I was able to find is from
pam. It says something like:
`pam_elogind[...] Failed to release session: No session 'cN' known`

Also, an issue was posted recently upstream about window managers not
being able to find Xauthority if XAUTHORITY environment variable is set.
See https://github.com/CanonicalLtd/lightdm/issues/68 . But I'm not sure
it's relevant her.

So if anyone has a lead, please share it! :)

Have a nice day,

L  p R n  d n