[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 99.02%. Comparing base (2741527) to head (a395fd5).

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/python-trio/trio/pull/3014?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_tools/gen\_exports.py](https://app.codecov.io/gh/python-trio/trio/pull/3014?src=pr&el=tree&filepath=src%2Ftrio%2F_tools%2Fgen_exports.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3Rvb2xzL2dlbl9leHBvcnRzLnB5) | `99.15% <ø> (ø)` | | | [src/trio/\_core/\_run.py](https://app.codecov.io/gh/python-trio/trio/pull/3014?src=pr&el=tree&filepath=src%2Ftrio%2F_core%2F_run.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX2NvcmUvX3J1bi5weQ==) | `99.07% <0.00%> (-0.31%)` | :arrow_down: | | [src/trio/\_core/\_io\_devpoll.py](https://app.codecov.io/gh/python-trio/trio/pull/3014?src=pr&el=tree&filepath=src%2Ftrio%2F_core%2F_io_devpoll.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX2NvcmUvX2lvX2RldnBvbGwucHk=) | `0.00% <0.00%> (ø)` | |