Update NEWS for 1.6.6

Finalize the release notes for OpenAFS 1.6.6

Change-Id: I9e86a1675d79a828beb39932d4cbf4e0668e549e
Reviewed-on: http://gerrit.openafs.org/10714
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>