new pre-freeze policy

[Greg Troxel <gdt%lexort.com@localhost>]

As you know there have been concerns about binary package availability
for stable branches.  This is a complicated problem and there has been a
lot of discussion around sub-pieces of the problem, most of it even
constructive!

One thing contributing to binary package unavailablity is lack of
stability of some packages, either the package not building reliably
everywhere, or the API being unstable.  This is compounded by chains of
packages; e.g. its hard to debug firefox if rust won't build.

PMC has discussed, and while the consensus is a bit rough in terms of
specific packages, has adopted a notion of early freeze for packages
with a history of trouble.  The dates are past for this quarter, but
this is also intended to apply to future quarters.  The
rapid-update/stable tension is not going away and this list is certainly
up for discussion for change over time -- it's meant to be today's plan
of record rather than being unchangeable.

It also attempts to document requirements for bulk build machines.

http://www.pkgsrc.org/quarterly/

tl;dr for this branch is:
  please no pre-freeze updates to boost, poppler, rust, firefox
  bulk builds for 8/9 need stable to get ld.elf_so fixes for rust