opt: enable ET mode on listener event-loop by default

[panjf2000]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 42.85714% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 79.47%. Comparing base (4033b47) to head (22f4be1).
Report is 1 commits behind head on dev.

Files	Patch %	Lines
acceptor_unix.go	46.15%	11 Missing and 3 partials ⚠️
engine_unix.go	0.00%	0 Missing and 1 partial ⚠️
eventloop_unix.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #585      +/-   ##
==========================================
+ Coverage   79.08%   79.47%   +0.39%     
==========================================
  Files          25       25              
  Lines        2252     2295      +43     
==========================================
+ Hits         1781     1824      +43     
  Misses        325      325              
  Partials      146      146              

Flag	Coverage Δ
unittests	79.47% <42.85%> (+0.39%)	⬆️

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