skeleton: sync /etc/shadow with /etc/passwd

Synchronize /etc/shadow with /etc/passwd:
  - remove "halt" and "uucp"
  - add "sys", "mail" and "www-data"

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>