OS_VERSION and NetBSD 10.0

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

The release of NetBSD 10.0 will cause problems in pkgsrc due to the
number of packages that attempt to evaluate the major version with
:M on OS_VERSION.

Is there any consensus on how we're going to deal with this yet?

If not, I propose introducing a OS_MAJOR_VERSION variable to the
mk/ infrastructure that only contains the first number.