use code points from x/sys/unix for PKTINFO syscalls

[marten-seemann]

Looks like there's no IPV6_{RECV}PKTINFO on darwin though.

[codecov]

Codecov Report

Merging #3094 (4f94be1) into master (e9ea484) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3094      +/-   ##
==========================================
- Coverage   85.29%   85.28%   -0.01%     
==========================================
  Files         131      131              
  Lines        9607     9607              
==========================================
- Hits         8194     8193       -1     
- Misses       1044     1045       +1     
  Partials      369      369              

Impacted Files	Coverage Δ
internal/utils/rand.go	62.50% <0.00%> (-12.50%)	⬇️

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