[PATCH] Port of ignore_ims_on_miss

This adds a directive configure_miss_revalidate
based closely on squid-2.7 ignore_ims_on_miss by Henrik.

The following changes are made:

* rename to cache_miss_revalidate
  Inverted the behaviour viewpoint from suppression to
  one of ability extension.

* fix RFC 2616 violation in 2.7 REFRESH
  In 2.7 any If-None-Match values added by Squid on a
  refresh HIT caused client header values to be dropped.

  The two sets of values need to be combined and delivered
  to the server to preserve '*' special case and allow a
  304 from server with ETag to be reliably authoritative
  in regards to the client delivered ETag values.
  Otherwise the server might return ambiguous succcess
  regarding Squid ETag when it should have sent 412
  regarding a client ETag.

* handle case where client is sending 'If-None-Match: *'

* remove HTTP_VIOLATIONS requirement
  This behaviour is permitted within the RFC 2616 update
  clarifications from IETF HTTPbis WG.

* improved documentation of behaviour and use cases

For now the default setting is to retain the current behaviour
of Squid-3 and Squid-2.7. Which is to pass If-* headers
through and continue MISS until e full fetch is requested
by a client.

Amos