Require `poll()` for UNIX based OSs and improve emulation for Windows

[Girgias]

Description

This would allow some simplification of the php_network.h header and of our emulation layer.

Initially found while working on #15511

[cmb69]

Makes sense to do this, but unless we are absolutely sure that all non Windows we are supporting support poll(), maybe it's better to postpone that to PHP next (8.5 or 9.0).

[Girgias]

This is definitely not urgent, the point of the issue is to have a reminder somewhere of this!

I also think it is better to wait for the next release as we're already in beta, and I am a bit vary to change such an aspect.