[PATCH] Send chunked responses if body size is unknown.

Send chunked responses if body size is unknown.

Apply HTTP chunked transfer encoding to the response body if all of the
following conditions are met:

* client claims HTTP version 1.1 or later support
* response does not have a Content-Length header already
* response does not use multipart/byteranges encoding
* connection is persistent

If we decide to send chunked reply, chunked_reply flag is set. Chunked
encoding is done in ClientSocketContext::packChunk(). The last-chunk is
sent only when clientReplyContext complete flag is set.

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This feature was requested to make Squid work with HTTP/1.1 clients that
can handle chunked responses but cannot handle connection closures in
the middle of a transaction sequence. The earlier version of the patch
(for Squid v3.1) was tested in production.

N.B. A bug in Squid may result in server-side code not treating
premature server-side connection termination as an error. That bug
results in Squid client-side sending a complete chunked response to the
client instead of omitting the last-chunk to indicate a truncated
response. Fixing that bug is outside this project scope (but we might
have a patch for it somewhere, I need to check).