MINOR: stream-int: add a new flag to mention that we want the connection to be killed

MINOR: stream-int: add a new flag to mention that we want the connection to be killed

The new flag SI_FL_KILL_CONN is now set by the rare actions which
deliberately want the whole connection (and not just the stream) to be
killed. This is only used for "tcp-request content reject",
"tcp-response content reject", "tcp-response content close" and
"http-request reject". The purpose is to desambiguate the close from
a regular shutdown. This will be used by the next patches.