Split out client protocols from protocol_t

http://www.squid-cache.org/bugs/show_bug.cgi?id=1763 has attached to it
a split out of the client protocol support from protocol_t.

This is fairly important to folk wanting to write new client protocol
modules - at the moment they need to perform shotgun surgery all through
the code base because the protocol gets listed as a protocol_t, which
also appears in the logic for request forwarding etc etc etc.

The patch is good enough to commit at the moment IMO.

Seeking thoughts on this.

-Rob

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