ICAP X-Authenticated-User support

Hello,

Here's a patch for Squid ICAP 2.5 from CVS to add the X-Authenticated-User
header described in :

  http://www.ietf.org/internet-drafts/draft-stecher-icap-subid-00.txt

This mimics the X-Client-IP header to provide more information regarding
the original request to the ICAP server. It works with both REQMOD and
RESPMOD (a small change was needed to REQMOD to keep the original
authentification when generating the new request so RESPMOD could use it
too when chained).

It adds two configurations options:
- "icap_send_auth_user" to enable or disable it;
- "icap_auth_scheme" to specify the auth_scheme to report to the ICAP
  server, since squid authentification mechanism doesn't know this.

I think this can be useful for others as well, and I'd be glad to know
what you think of it.

Gaël.

-- 
Gaël Roualland -+- gael.roualland@oleane.net