A few changes related to how the ICAP request header is built:

A few changes related to how the ICAP request header is built:

 - Added 'icap_persistent_connections' directive to squid.conf to turn
   on/off ICAP server connection reuse.

 - Renamed 'icap_send_auth_user' directive to 'icap_send_client_username'

 - Removed 'icap_auth_scheme' directive since it is not currently used.

 - Fixed ICAPModXact::makeRequestHeaders() to add X-Client-IP and
   X-Client-Username only when those options are enabled in squid.conf.