Removing interix support

[coypu%sdf.org@localhost]

Hi,

Interix was the old way of getting POSIX things working on Windows.
Nowadays there are other, better ways of doing it.

Interix occasionally required invasive changes to packages, like openssh
before https://v4.freshbsd.org/commit/netbsd/pkgsrc/Q6ZOqBt348zsPwlB
It wasn't a very good POSIX platform.

I doubt anyone tested such changes in a while and likely any real
interix user will encounter issues with having a too old compiler.

Proposal: we stop claiming interix support and gradually remove it from
places.

Thoughts?