Changes to NetBSD release support policy

[Soren Jacobsen <snj%NetBSD.org@localhost>]

The NetBSD release engineering team is announcing a new support policy
for our release branches.  This affects NetBSD 8.0 and subsequent major
releases (9.0, 10.0, etc.).  All currently supported releases (6.x and
7.x) will keep their existing support policies.

Beginning with NetBSD 8.0, there will be no more teeny branches (e.g.,
netbsd-8-0).

This means that netbsd-8 will be the only branch for 8.x and there will
be only one category of releases derived from 8.0: update releases.  The
first update release after 8.0 will be 8.1, the next will be 8.2, and so
on.  Update releases will contain security and bug fixes, and may contain
new features and enhancements that are deemed safe for the release
branch.

With this simplification of our support policy, users can expect:
- More frequent releases
- Better long-term support (example: quicker fixes for security issues,
  since there is only one branch to fix per major release)
- New features and enhancements to make their way to binary releases
  faster (under our current scheme, no major release has received more
  than two feature updates in its life)

We understand that users of teeny branches may be concerned about the
increased number of changes that update releases will bring.
Historically, NetBSD stable branches (e.g., netbsd-7) have been managed
very conservatively. Under this new scheme, the release engineering team
will be even more strict in what changes we allow on the stable branch.
Changes that would create issues with backwards compatibility are not
allowed, and any changes made that prove to be problematic will be
promptly reverted.

The support policy we've had until now was nice in theory, but it has
not worked out in practice.  We believe that this change will benefit
the situation for vast majority of NetBSD users.