[PATCH] Bug 3342: username token for external ACL without triggering auth validation

Original design by "arronax28".

This adds a %un token (different to %LOGIN and %EXT_USER) which passes
any pre-known username details to the external ACL helper. But does not
trigger or require authentication verifications.

This will not process auth headers if presented but not yet
authenticated. But it will allow IDENT and external ACL out-of-band
authorization usernames to be sent to the helper.

On the upside it will solve some of the cases where people want to
process usernames without accidental auth challenges.

On the downside I am expecting some small amount of confusion as admin
send HTTP auth headers and expect Squid to magically understand them
without doing any auth processing.

Amos