opt: refine the code of I/O handlers

[codecov[bot]]

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 :warning:
engine_unix.go	85.71%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/panjf2000/gnet/pull/586/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/panjf2000/gnet/pull/586/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `78.83% <76.66%> (-0.13%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan#carryforward-flags-in-the-pull-request-comment) to find out more.

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