BUG/MINOR: http-ana: Properly detect client abort when forwarding the response

BUG/MINOR: http-ana: Properly detect client abort when forwarding the response

During the response payload forwarding, if the back SC is closed, we try to
figure out if it is because of a client abort or a server abort. However,
the condition was not accurrate, especially when abortonclose option is
set. Because of this issue, a server abort may be reported (SD-- in logs)
instead of a client abort (CD-- in logs).

The right way to detect a client abort when we try to forward the response
is to test if the back SC was shut down (SC_FL_SHUT_DOWN flag set) AND
aborted (SC_FL_ABRT_DONE flag set). When these both flags are set, it means
the back connection underwent the shutdown, which should be converted to a
client abort at this stage.

This patch should be backported as far as 2.8. It should fix last strange SD
report in the issue #2749.