POSIX poller: add support for select, and for choosing the poller

[gdamore]

Some platforms or configurations may not have more modern options like kqueue or epoll, or may be constrained by policy.

fixes #

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[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.99%. Comparing base (d83f5ae) to head (6c949de).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/platform/posix/posix_pollq_kqueue.c	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1996      +/-   ##
==========================================
- Coverage   82.07%   81.99%   -0.08%     
==========================================
  Files          95       95              
  Lines       24033    24033              
  Branches     3167     3167              
==========================================
- Hits        19725    19706      -19     
- Misses       4235     4256      +21     
+ Partials       73       71       -2     

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