[PREVIEW] Various ACL fixes around matchAclList

Hello,

    The attached work-in-progress patch attempts to fix most of the bugs
discussed in recent ACL-related squid-dev threads ("DUNNO state and
implicit ACLs" and "Handle ACLs that are neither denied nor allowed").
Comments welcomed!

Done:

 * Removed ACLChecklist::lastACLResult(). It was doing nothing but
   duplicating nodeMatched value as far as I could tell.

 * Move away from setting the "default" (and usually wrong) "current"
   answer and then sometimes adjusting it. Set the answer only when
   we know what it is.

 * Correctly handle cases where no rules were matched and, hence, the
   keyword/action of the last seen rule (if any) has to be "reversed".

 * Do not ignore non-allow/deny outcomes of rules in fastCheck().

 * Streamline and better document ACLChecklist::matchAclList()
   interface. Use it in a more consistent fashion.

 * Better document and restrict ACLChecklist::matches() outcomes;
   list the ones we actually support. Assert on unsupported outcomes
   (for now).

TODO:

 * Remove ProxyAuthNeeded class. It is an async state that does not
   perform async operations and, hence, is not needed.

 * Move IdentLookup::checkForAsync() connection check into
   ACLIdent::match() to avoid creating an async state that is not
   needed.

 * Rename currentAnswer() to finalAnswer(). We probably never change the
   "current" answer any more.

 * Testing, trunk port, and polishing.

 * Detail all patch changes.

Cheers,

Alex.