Please test/review pbulk chroot patch

[Jonathan Perkin <jperkin%joyent.com@localhost>]

I've written a patch to make it simpler for users to get going with
chrooted concurrent pbulk builds, and would appreciate wider testing
and review.

The patch is here:

  https://github.com/joyent/pkgsrc/commit/0961b41d8660a5087aef913f50deab7f127c01f8

On NetBSD at least, the tl;dr for performing a full bulk build for an
/opt/pkg prefix in 4 chroots after applying this patch would be:

  $ cat >/tmp/mk-frag.conf <<EOF
  PBULK_CACHE_DIRECTORY=/mnt/bulklog/cache
  WRKOBJDIR=/var/tmp
  EOF

  $ PREFIX=/opt/pkg sh mk/pbulk/pbulk.sh -c /tmp/mk-frag.conf -C 4

  $ /usr/pbulk/bin/bulkbuild

Further details are in the commit message.  This uses
pkgtools/mksandbox for the chroot creation so would appreciate wider
platform testing too.  There is at least one issue on EL7 Linux with
systemd that I haven't applied yet, patch for that is available here:

  https://gist.github.com/jperkin/7946cc8cf810cbca709f8bbf7eb1af68

Thanks!

-- 
Jonathan Perkin  -  Joyent, Inc.  -  www.joyent.com