HTCP enhancements

The attached patch adds some enhancements to squid 2.HEAD's HTCP
support. These are:

- Properly process CLR requests.
- Allow peers to have no CLRs, only CLRs, or only CLRs not related to
PURGE requests sent to them.
- Allow CLRs to be forwarded to other peers.

I'm planning to commit this patch on Friday barring any objections.

-- 
Benno Rice
benno_at_jeamland.net