additional software: polish backend

• when the notification fails, logging stdout/stderr/returncode of the tails-notify-user
  • filter out empty lines from the conf file: else, APT is called in a way that makes it crash
  • explain the user that most APT lists expire after a week or so, and that will make offline packages installation fail after a while
  • move the args we always want to pass to apt-get (--yes and --quiet) into _launch_apt_get.
  • verify the need of a custom apt-get PATH and add a comment to explain if it’s really needed.

Fixed in feature/remember_installed_packages, for Tails 0.20.

Parent Task: #5551 (closed)

Original created by @tails on 5951 (Redmine)