pam: Export a pam_sm_close_session() function entry point

Some PAM callers want this even though we don't do anything
interesting in here.

https://bugzilla.gnome.org/show_bug.cgi?id=726245
pam: Allow unlock_keyring() to be called with a NULL password

This happens when doing auto-login. The various side effects
of unlock_keyring (including setting *need_daemon) are valuable
even in the cases where password is NULL.

Add a test that checks that the daemon starts as expected when
the user did not authenticate.

http://bugzilla.gnome.org/show_bug.cgi?id=726245