[PATCH] reload into If-None-Match

This patch sends an If-None-Match request, when we need to re-validate
if a cached object which has a strong ETag is still valid.

This is also done in the cases an HTTP client request contains HTTP
headers prohibiting a from-cache response (i.e., a "reload" request).
The use of If-None-Match request in this context violates RFC 2616 and
requires using reload-into-ims option within refresh_pattern squid.conf
directive.

The exact definition of a "reload request" and the adjustment/removal of
"reload" headers is the same as currently used for reload-into-ims
option support. This patch is not modifying that code/logic, just adding
an If-None-Match header in addition to the IMS header that Squid already
adds.

This is a Measurement Factory Project