enhance RADIUS client with group enumeration capabilities for ACLing

[thorstenrood]

I'd like to have the RADIUS client to parse for a configurable return attribute on grant message and its string content, maybe in delimited format, to gain group membership for RADIUS-based authentication to squid and use with along with the squid-ACL module. As of now, with RADIUS it's just a grant/deny all-in but nothing granular. ldap is not always the desired or available option. ;-)