environ: limit X11 environment to inside X11
Revert "environ: limit X11 environment to inside X11"

On the second thought, it'll break when $DISPLAY becomes optional for
libX11, and it'll break when I use Wayland

(...yeah, as if any of these are going to matter in Wayland)

Aaaanyway. Let's stick to the simpler version.

This reverts commit 6f240e9847e721fc3b4cfa80cb241bdacf6701a3.
environ: remove obsolete Windows_NT check

I'm not going to run halfassed X sessions within Cygwin any time soon.