[MERGE] Finish forward-porting HTCP enhancements from squid 2.HEAD.

This is a forward-port of the HTCP changes I made to squid 2.HEAD.

Changes include:

- Ability to send HTCP CLR requests when objects are invalidated or purged from
  the cache.
- Config logic to allow the following:
        - HTCP peers who ONLY receive CLR messages from us.
        - HTCP peers who NEVER receive CLR messages from us.
        - HTCP peers who NEVER receive CLR messages from us for PURGE requests.
        - HTCP peers who are forwarded CLR messages we receive.
- Code to support all of the above.