[LightDM] Authentication with no passwords

[David Edmundson]

Log for user foo which should have no password having run "sudo
gpasswd foo nopasswdlogin" before.

[+5.13s] DEBUG: Greeter start authentication for foo
[+5.13s] DEBUG: Started session 15571 with service 'lightdm', username 'foo'
[+5.14s] DEBUG: Session 15571 authentication complete with return
value 7: Authentication failure
[+5.14s] DEBUG: Authenticate result for user foo: Authentication failure
[+5.14s] DEBUG: Session 15571 exited with return value 1

It's definitely doing something different, because for a regular user,
I get a prompt, and a 3 second delay before authentication completes.

[+54.74s] DEBUG: Greeter start authentication for david
[+54.74s] DEBUG: Started session 15831 with service 'lightdm', username 'david'
[+54.77s] DEBUG: Session 15831 got 1 message(s) from PAM
[+54.77s] DEBUG: Prompt greeter with 1 message(s)
[+54.77s] DEBUG: Continue authentication
[+57.17s] DEBUG: Session 15831 authentication complete with return
value 7: Authentication failure
[+57.17s] DEBUG: Authenticate result for user david: Authentication failure
[+57.17s] DEBUG: Session 15831 exited with return value 1

Dave