Add support for TCP_QUICKACK
TCP_QUICKACK is a Linux setsockopt() option. When a socket is in quickack mode, it acks each TCP packet immediately rather than delaying in the hopes that it might be able to ack multiple packets at once.
Linux has had support for this option for ~20 years. While it doesn't seem to be widely used, I ran into a vendor-provided, closed-source tool that uses it. When running the tool under Illumos without enabling this mode, the increased latency caused a significant performance hit. I was able to work around the issue by using ndd to set the deferred-ack limit to 0. Using a per-socket mechanism rather than system-wide will reduce overall chattiness, allowing the system to remain a good network citizen.