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

[codecov[bot]]

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 :warning:
engine_unix.go	0.00%	0 Missing and 1 partial :warning:
eventloop_unix.go	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/panjf2000/gnet/pull/585/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/585/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andy+Pan) | `79.47% <42.85%> (+0.39%)` | :arrow_up: | 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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