Presence of modular X package in native X package sets

[nia <nia%NetBSD.org@localhost>]

Hello,

Native vs. modular X often seems to be a point of confusion for new
NetBSD users, and I often have to explain that, by default, binary
packages only work with native X. Users coming from Linux are often
still under the impression that they can "install drivers" from
pkgsrc by virtue of them being there.

I think the UX would improve a lot if we removed this source of
confusion by making modular xorg packages NO_BIN_ON_FTP when
the xorg type is set to native (another option would be adding
SKIP_, but do we want bulk build logs for this mixed ABI-incompatible
configuration?).

Since I'm aware of a few users doing stuff like using xterm from
pkgsrc with native X from base, I think we should continue supporting
this use case (because it's relatively harmless) but reserve the
restrictions for server and library packages. The libraries should
not be mixed because NetBSD's X has different shared library versions.

Thoughts?