macos: use pkgbuild to build the package on 10.10/10.11

macos: use pkgbuild to build the package on 10.10/10.11

PackageMaker is no longer part of OS X. As a result, it
is not possible to build the package on OS X 10.10 and
OS X 10.11 using the existing code.

To solve this problem, a new script, along with a couple
of new files, are provided.

- pkgbuild.sh

This script uses the command line tools pkgbuild and
productbuild to build the package on OS X 10.10 and
OS X 10.11. By default, the package built by this
script will not be signed. Optionally, the package
might be signed.

- Distribution.xml

This file is nothing more than an XML file used by
productbuild. It is mainly used to configure how the
installer will look and behave.

- conclusion.txt

Contains the text that is displayed by Installer at
the end of the installation process. Only used by
El Capitan and further.

- Uninstall.14.15

This script can be used by OS X 10.10/10.11 users
to uninstall OpenAFS.

Notes:

- This work is based on a patch made by Brandon Allbery
  <ballbery@sinenomine.net> with fixes and updates from
  Andrew Deason <adeason@dson.org>.

- El Capitan and further prevent us from touching
  /usr/bin directly. As a result, /opt is used.

- If the package is not signed, the user will have
  to disable the OS X security protections. Otherwise,
  the client will not work.

- Now we have two different scripts to build the
  package on OS X. For OS X 10.10 and newer versions,
  pkgbuild.sh will be used. For older versions,
  the existing buildpkg.sh will be used.

Change-Id: If8320666c553b82af450c0263f5e80a00c33e3b8
Reviewed-on: https://gerrit.openafs.org/12239
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>