set the UDP receive buffer size on Windows

[marten-seemann]

Fixes #2837.

[codecov]

Codecov Report

Merging #2896 (9d93074) into master (69158cf) will decrease coverage by 0.08%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master    #2896      +/-   ##
==========================================
- Coverage   85.77%   85.70%   -0.08%     
==========================================
  Files         133      133              
  Lines        9187     9194       +7     
==========================================
- Hits         7880     7879       -1     
- Misses        959      963       +4     
- Partials      348      352       +4     

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <ø> (ø)
conn_windows.go	60.00% <60.00%> (ø)
internal/flowcontrol/connection_flow_controller.go	100.00% <100.00%> (ø)
internal/flowcontrol/stream_flow_controller.go	100.00% <100.00%> (ø)
packet_handler_map.go	72.09% <0.00%> (-0.93%)	⬇️

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[mvdan]

SGTM, but you should really not use the syscall package directly on linux/darwin and other unixes. Just like you use x/sys/windows for windows, use x/sys/unix for those, which is actively maintained.

The only time it makes sense to use syscall is for std APIs which are stuck with syscall forever, since x/sys/... came after.

[marten-seemann]

Good point. Will do so in a separate PR.