libfetch: support TCP_CORK
Unfortunately libfetch operates on raw sockets and is sending each HTTP request line using separate syscall which causes the HTTP request to be sent as multiple packets over the wire in most configurations. This is not good for performance, but can also cause subtle breakage if there's DPI firewall that does not get the Host header. Incidentally, it seems that on BSDs libfetch already sets TCP_NOPUSH optimize the packetization. This commit adds same logic for using TCP_CORK if available. When using TCP_CORK there is no requirement to set TCP_NODELAY as uncorking will also cause immediate send. Keep TCP_NODELAY in the fallback codepaths. Long term, it might make sense to replace or rewrite libfetch to use application level buffering.
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