opt: only enable SO_REUSEPORT on Linux and FreeBSD

[panjf2000]

Fixes #579

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.80%. Comparing base (bf7121d) to head (9acaad8).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #580      +/-   ##
==========================================
- Coverage   78.91%   78.80%   -0.11%     
==========================================
  Files          25       25              
  Lines        2248     2251       +3     
==========================================
  Hits         1774     1774              
- Misses        327      331       +4     
+ Partials      147      146       -1     

Flag	Coverage Δ
unittests	78.80% <100.00%> (-0.11%)	⬇️

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