opt: refine the code of I/O handlers

[panjf2000]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 76.66667% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 78.83%. Comparing base (d473f26) to head (aee4817).

Files	Patch %	Lines
acceptor_unix.go	50.00%	6 Missing ⚠️
engine_unix.go	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #586      +/-   ##
==========================================
- Coverage   78.96%   78.83%   -0.13%     
==========================================
  Files          25       23       -2     
  Lines        2258     2249       -9     
==========================================
- Hits         1783     1773      -10     
- Misses        327      329       +2     
+ Partials      148      147       -1     

Flag	Coverage Δ
unittests	78.83% <76.66%> (-0.13%)	⬇️

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