Added support for squiguard username handling when URL requests are submitted via Websense v4 from Cisco ASA using user-identity (Identity Firwall)

* added support of username passed via Websense v4 TCP from Cisco ASA + Identity firewall
* added -u option so existing IP-based URL checks using sg don't break (only available with websense frontend)
* added additional debug output for troubleshooting

currently it only supports the squidguard backend as I needed this to work with sg ldapusersearch lookups (it already works fine with ldapipsearch lookups). Should be easy to extend to proxy checks if desired; I might try to do this if I need to use a proxy backend, but I don't think I can beat the performance of using straight squidguard.