Fix default user handling.

Fix default user handling.

* Handle the case in which an user is present but the default user could not
  be created (e.g. when configuring another user at build time in the
  chroot). If the default user could not be created or assigned, disable
  autologins (which previously just fails leaving without console login
  capabilities), and propagate that information to the scripts thinking that a
  default user will always be available.
  Note for live-helper documentation: The latter case could leave the system
  accessible only if root password is set in other ways or via ssh rsa/dsa
  passwordless accesses.

Signed-off-by: Marco Amadori <amadorim@vdavda.com>