[WIP] devpoll (solaris) support

[CoolCat467]

This pull request is attempting to add /dev/poll (devpoll)-based IO polling. As of writing this is currently untested and I don't really have experience working with this sort of thing, but I'm going to try anyways! At the moment, I've basically stolen everything the epoll system does, and reading about the differences I don't really see a whole lot.

Mostly for myself but maybe helpful resources:

• https://docs.oracle.com/en/operating-systems/solaris/oracle-solaris/11.4/prog-interfaces/using-poll-driver.html
• https://docs.oracle.com/cd/E88353_01/html/E37851/poll-4d.html
• https://www.techrepublic.com/article/using-the-select-and-poll-methods/

EDIT: Might not actually go anywhere because of issues with getting a solaris runner in CI

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 111 lines in your changes missing coverage. Please review.

Project coverage is 99.42061%. Comparing base (d65d3f6) to head (ab18c46).

Files with missing lines	Patch %	Lines
src/trio/_core/_io_devpoll.py	0.00000%	108 Missing :warning:
src/trio/_core/_run.py	0.00000%	3 Missing :warning:

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3014         +/-   ##
====================================================
- Coverage   100.00000%   99.42061%   -0.57940%     
====================================================
  Files             124         125          +1     
  Lines           19047       19158        +111     
  Branches         1287        1301         +14     
====================================================
  Hits            19047       19047                 
- Misses              0         111        +111     

Files with missing lines	Coverage Δ
src/trio/_tools/gen_exports.py	100.00000% <ø> (ø)
src/trio/_core/_run.py	99.70760% <0.00000%> (-0.29241%)	:arrow_down:
src/trio/_core/_io_devpoll.py	0.00000% <0.00000%> (ø)

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