use golang.org/x/sys/unix instead of syscall

[marten-seemann]

As suggested by @mvdan in #2896 (review). An excellent suggestion, as it allows us to get rid of two OS-specific files.

[dswarbrick]

These constants exist in the unix package also, for consistency.

Suggested change

	size, serr = unix.GetsockoptInt(int(fd), syscall.SOL_SOCKET, syscall.SO_RCVBUF)
	size, serr = unix.GetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_RCVBUF)

[marten-seemann]

Thank you, missed that one!

[codecov]

Codecov Report

Merging #2927 (f59cd92) into master (5a0ce24) will increase coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2927      +/-   ##
==========================================
+ Coverage   86.24%   86.61%   +0.37%     
==========================================
  Files         133      132       -1     
  Lines        9143     9378     +235     
==========================================
+ Hits         7885     8122     +237     
+ Misses        910      907       -3     
- Partials      348      349       +1     

Impacted Files	Coverage Δ
conn_ecn.go	70.97% <100.00%> (ø)
conn_windows.go	60.00% <100.00%> (ø)
qlog/frame.go	98.47% <0.00%> (+0.20%)	⬆️
qlog/event.go	98.23% <0.00%> (+0.57%)	⬆️
qlog/qlog.go	96.96% <0.00%> (+1.29%)	⬆️
qlog/packet_header.go	98.46% <0.00%> (+3.72%)	⬆️
qlog/trace.go	96.77% <0.00%> (+5.47%)	⬆️

---

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[marten-seemann]

Looks like I was too optimistic here.
On Linux, the Type of the TOS control message is unix.IP_TOS, whereas on Darwin it's unix.IP_RECVTOS 🤦🏻‍♂️:
https://github.com/lucas-clemente/quic-go/blob/5a0ce242654c153128ca8679e8fe19fc3be966d3/conn_ecn.go#L96-L105

That means we'll still need OS-specific files to set the msgTypeIPTOS constant.