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.

[Girgias]

Just rediscovered this, maybe @petk has any knowledge about the availability of poll on other platforms?

[petk]

It's widely available. In practice it won't be possible to find a POSIX system without it.

It was once part of the XSI (so some systems lacked it - some very old Solaris versions probably, IRIX, some very old BSD-based systems and similar) but now it is moved to base POSIX:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/poll.html

I'm also collecting all such similar POSIX functionality here: https://github.com/petk/php-build-system/blob/8af7cf3d50b738764d5cd2aa22f3775174796f13/cmake/cmake/platforms/Unix.cmake
for which it can be assumed that they are available on all POSIX systems. If I find some system that where it would be still relevant to check all of these, I'll notice it sooner or later.