Add support for SO_REUSEPORT to vtgate (#19168)

[rjlaine]

Description

Backports the following PR:
vitessio#19168

This will be used by vtgateproxy for hot-reloading.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 56.75676% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.77%. Comparing base (946a513) to head (0d3c8c2).

Files with missing lines	Patch %	Lines
go/vt/vtgate/plugin_mysql_server.go	0.00%	7 Missing ⚠️
go/netutil/reuseport_unix.go	76.47%	4 Missing ⚠️
go/vt/servenv/listen.go	62.50%	3 Missing ⚠️
go/mysql/server.go	66.66%	1 Missing ⚠️
go/vt/servenv/run.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           slack-22.0     #796      +/-   ##
==============================================
- Coverage       69.77%   69.77%   -0.01%     
==============================================
  Files            1605     1607       +2     
  Lines          213999   214025      +26     
==============================================
+ Hits           149324   149341      +17     
- Misses          64675    64684       +9     

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