client side internal request support

As Adrian commented, this little puppy needs some help getting into
HEAD.

The attached patch splits most of the client side functionality into
three sections:
Request management
Reply Management
'external' or socket related and 'core'

It also enables hookable code to plug into the client side
infrastructure after all the Vary, Range and other HTTP mechanics are
dealt with. This is particularly useful for ESI, but may also be useful
for other projects that need to make RFC conformant requests from within
squid. Finally, it's a first step towards cleaning up the mess that has
accrued infront of the store (many of the _reply functions and _request
functions are candidates for enventual inclusion in the store manager
code).

I get identical results with this code, and with HEAD with polygraph.
AFAIK I've caused no regressions.

So, anyone for a little bit of peer review?

I'd really like to get this into HEAD asap...

Cheers,
Rob