Poll(2) implementation follow-ups

[Thomasdezeeuw]

The implementation was initially added in #1687 (issue: #1604), but it's experimental. Below is a small list of future improvements/work:

• Optimise some allocations as well as locking: poll: Optimize locks and allocations in the critical path #1701.
• Determine how to handle platforms without POLLRDHUP, e.g. macOS, as it makes our some tests fail.
• Run the CI on more platforms, currently it only runs Linux.
• Add support for ESP-IDF: Add ESP-IDF framework support #1692.
• Add support for Haiku: Mio build broken on Haiku, no kpoll/kqueue #1472 (already closed) and discussion in Poll selector #1602 (also already closed).
• Document the implementation details, in e.g. Poll, Event, etc.
• Document the flags on how to use the implementation (while experimental): Doc unsupported cfg flags #1684.