unlockDialog: better confirmation button label

If I click on "Not listed?" in the login screen, I come to a username
field with two buttons: "Cancel" and "Sign In".

Clicking on "Sign In" doesn't actually sign me in though - it takes me
to the login entry. It would be better to rename "Sign In" to "Next" for
the username stage, therefore.

Gdm emit a signal to ask a question or a secret, but we can not know if
this is the last authentication question, hence we only use "Sign In"
for secret questions which improve the situation a lot.

https://bugzilla.gnome.org/show_bug.cgi?id=687656