[PATCH] Compliance: Forward 1xx control messages to clients

Compliance: Forward 1xx control messages to clients that support them.

Forward 1xx control messages to all HTTP/1.1 clients and to HTTP/1.0
clients that sent an Expect: 100-continue header unless the 1xx message
fails the optional http_reply_access check described below.

RFC 2616 requires clients to accept 1xx control messages, even if they
did not send Expect headers. However, 1xx control messages prohibited by
http_reply_access are ignored and not forwarded. This can be used to
protect broken HTTP/1.1 clients or naive HTTP/1.0 clients that
unknowingly forward 100-continue headers, for example. Only fast checks
are supported at this time.

The patch removes ignore_expect_100 squid.conf option and the
corresponding code because

   - the reasons to treat 100-continue specially have changed since we
     can now forward 1xx responses;

   - rejection of 100-continue request can still be done using a
     combination of the existing http_access and deny_info options;

   - hiding of 100-continue header from next hops can still be done using
     an existing request_header_access option;

   - 100 (Continue) responses can be hidden from clients using
     http_reply_access check described above.

We still respond with 417 Expectation Failed to requests with
expectations other than 100-continue.

Implementation notes:

We forward control messages one-at-a-time and stop processing the server
response while the 1xx message is being written to client, to avoid
server-driven DoS attacks with large number of 1xx messages.

1xx forwarding is done via async calls from HttpStateData to
ConnStateData/ClientSocketContext. The latter then calls back to notify
HttpStateData that the message was written out to client. If any one of
the two async messages is not fired, HttpStateData will get stuck unless
it is destroyed due to an external event/error. The code assumes such
event/error will always happen because when
ConnStateData/ClientSocketContext is gone, HttpStateData job should be
terminated. This requires more testing/thought, but should still be
better than not forwarding 1xx messages at all.

----
I believe the current approach addresses Henrik's comments regarding 
forwarding to HTTP/1.0 clients without drop_expect_100. An admin would 
be able to prohibit such forwarding (globally or selectively using 
http_reply_access). If HTTPbis changes the specs later, we can change 
the default.
Thank you,
Alex.