ACL refactoring

I've been babying along an overhaul of the ACL code.

I've tackled and solved all the forseeable problems, although I haven't
migrated all the ACL types to the new framework. The code is fully
functional though - everything is still there :].

I'd like to get this into 3.0, as doing MFC's against it once it's in
HEAD would be annoying....

Any objections? (I know, sight-unseen :[).

The important changes are:
A registry of acl types that is populated by the ACL classes themselves.
ACL's are derived from a common base class, and implement methods for
matching, cloning, and parsing / cache mgr dumping.
ACLChecklist uses a state object to allow async lookups to take place
whilst remaining ignorant of the ACL type doing the lookup.

One neat sideeffect of all this is that acl.cc doesn't need to change at
all when new ACL types (i.e. ACL_EXTERNAL) are defined. This means that
simply adding a new source file to Makefile.am/.in and recompiling is
all thats involved in extending the ACL types.

Rob

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