[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]

Codecov Report

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

Project coverage is 99.02%. Comparing base (2741527) to head (a395fd5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3014      +/-   ##
==========================================
- Coverage   99.63%   99.02%   -0.62%     
==========================================
  Files         120      121       +1     
  Lines       17855    17966     +111     
  Branches     3212     3234      +22     
==========================================
  Hits        17790    17790              
- Misses         46      157     +111     
  Partials       19       19              

Files	Coverage Δ
src/trio/_tools/gen_exports.py	99.15% <ø> (ø)
src/trio/_core/_run.py	99.07% <0.00%> (-0.31%)	⬇️
src/trio/_core/_io_devpoll.py	0.00% <0.00%> (ø)