Add support for Windows Registered I/O

[DBLouis]

This allows to create socket with the WSA_FLAG_REGISTERED_IO flag.

[Thomasdezeeuw]

One more change then it LGTM

[Thomasdezeeuw]

Should I also simplify it for the existing custom flag?

No, because that is used with the all feature being enabled, that's the reason the code is written the way it is.

[DBLouis]

Should I also simplify it for the existing custom flag?

No, because that is used with the all feature being enabled, that's the reason the code is written the way it is.

I don't get it sorry. Also I put the new function with the all feature flag too, should I remove it?

[Thomasdezeeuw]

@DBLouis you need to revert all changes to _no_inherit and no_inherit. We do it this way because no_inherit should only be public if the all feature is set. However, internally we also use the function, so we added _no_inherit that is always available.

[DBLouis]

That makes sense. So I revert the changes for no_inherit, but for registered-io I keep only one function. Should it be feature gated though? It is a windows only flag and a bit niche.

[Thomasdezeeuw]

Should it be feature gated though? It is a windows only flag and a bit niche.

That's exactly why it should be feature gated.