Create and unlock login keyring for passwordless accounts

Without this change, passwordless accounts (which are not created by gnome-initial-setup) do not automatically get a login keyring, so launching an application (such as Chromium) which uses the login keyring causes the user to be prompted to create one.

The stock version of gnome-control-center does not permit creating passwordless accounts, but there's nothing to stop an administrator creating one the old-fashioned way.

Endless's fork of gnome-control-center adds UI to create passwordless accounts, which makes this situation more likely.

Endless also patches gnome-initial-setup to allow creating the initial administrator account with no password, but that account doesn't hit this problem because gnome-initial-setup manually creates a keyring and sets its passphrase to "" when it's done. (This keyring is not unlocked at login, but it is silently unlocked when it's needed, which is equivalent from the user's perspective.)

This is a continuation of c5da2060 where @matthiasc tried to make this work for gnome-initial-setup's benefit. The corresponding change in gnome-initial-setup was subsequently reverted gnome-initial-setup@b249c898 because that change didn't work – I think this is why.