[core] PamBackend does not honor ignore_username_case

When users login with PAMBackend, and set:

force_username_lowercase=true
ignore_username_case=true

with username:

Admin12345

It should force that to admin12345 and realize the user exists already. Instead it logs them in as Admin12345 and creates another user causing Job and Filebrowser issues as their home dir is /user/admin12345.