login: track login class (i.e. one of "user", "greeter", "lock-screen") for each...

login: track login class (i.e. one of "user", "greeter", "lock-screen") for each session

This introduces the new PAM environment variable XDG_SESSION_CLASS. If
not set, defaults to "user".

This is useful for apps that want to distuingish real user logins from
"fake" ones which just exist to show a gdm login screen or a lock
screen.