squid: create a user/group

Even though squid uses nobody/nogroup it ain't good for security if
every daemon around uses it, specially since squid is used as a caching
proxy most of the time and that would mean other daemons/scripts run as
nobody would have access to potentially sensitive information.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>