Update NEWS for 1.6.8

Finalize the 1.6.8 release notes

Change-Id: Ic97d1767ff168b538ce69c15aa8c7f0af4607b1b
Reviewed-on: http://gerrit.openafs.org/11136
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Make OpenAFS 1.6.8

Update version strings for the 1.6.8 release

Change-Id: If74aea398c0210807d8e5a1d9236a4c6242ac23f
Reviewed-on: http://gerrit.openafs.org/11137
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>