installer: Fix handling of user password mismatches.

Previously, if we had a password mismatch, the 'password' field would
end up containing a <user> record instead of the actual password.

* gnu/installer/newt/user.scm (confirm-password): Make TRY-AGAIN
optional and adjust docstring.
(run-user-add-page): Move 'confirm-password' call one level higher.