BUG/MEDIUM: tcp-checks: disable quick-ack unless next rule is an expect

BUG/MEDIUM: tcp-checks: disable quick-ack unless next rule is an expect

Using "option tcp-checks" without any rule is different from not using
it at all in that checks are sent with the TCP quick ack mode enabled,
causing servers to log incoming port probes.

This commit fixes this behaviour by disabling quick-ack on tcp-checks
unless the next rule exists and is an expect. All combinations were
tested and now the behaviour is as expected : basic port probes are
now doing a SYN-SYN/ACK-RST sequence.

This fix must be backported to 1.5.