Turn on the direct ACK on every other segment.

Turn on the direct ACK on every other segment.

This is a backout of rev 1.366 which turned this feature off.
Although sending less ACKs makes TCP faster if the CPU is busy with
processing packets, there are corner cases where TCP gets slower.

Especially OpenBSD 6.8 and older has a maxbust limitiation that
scales badly if the other side sends too few ACKs.  Also regress
test relayd run-args-http-slow-consumer.pl uses strange socket
buffer sizes that triggers slow performance with the new algorithm.

For OpenBSD 6.9 release switch back to 6.8 delayed ACK behavior.

discussed with deraadt@ benno@ claudio@ jan@