[PATCH] external ACL non-caching update - round 2

Admin configure ttl=0 and/or negative_ttl=0 to prevent Squid storing the
ACL lookup results. The problem is that results still get cached and
re-used for the grace= period or one second, whichever is larger.

Also, in the event where two or more requests with identical details
needing to be looked up at the same time there is an optimization which
will merge and share one lookup result for all these requests.

In most situations this result sharing is beneficial, however when a
unique result is wanted it can cause problems.

This patch makes ttl=0 and negative_ttl=0 prevent their respective OK
and ERR results from being stored into the helper result cache. Sharing
is still performed for overlapping duplicate requests.

When cache=0 is configured, no caching or sharing of results is
performed at all.

Amos