BUG/MINOR: http: don't report client aborts as server errors

BUG/MINOR: http: don't report client aborts as server errors

If a client aborts with an abortonclose flag, the close is forwarded
to the server and when server response is processed, the analyser thinks
it's the server who has closed first, and logs flags "SD" or "SH" and
counts a server error. In order to avoid this, we now first detect that
the client has closed and log a client abort instead.

This likely is the reason why many people have been observing a small rate
of SD/SH flags without being able to find what the error was.

This fix should probably be backported to 1.4.