Change the shell for services in /etc/passwd to /bin/false

A lot of user accounts which are only used for services (games, lp, mail, news.. etc) have their shell set to /bin/sh in the password file /etc/passwd. For security reasons it might be wise to change them default to a shell like /bin/false. As far as i see this won't affect the operation of the system in any way except a small security improvement.