Bug#867311: How ticket renewal works on Jessie (gdm3-3.14.1-7)

[John Hughes]

gdm3 sends a StartReauthentication message to the gdm-session-worker.
gdm-session-worker creates a ReauthenticationRequest.

reauthentication_request_new creates a GdmSession with the environment 
from PAM (including KRB5CCNAME)

session_worker_job starts a new gdm-session-worker with that environment

the new worker does the reauthentication, then pam_sm_setcred: entry 
(reinit) which refreshes the ticket cache.