Make OpenAFS 1.6.21pre1

Make OpenAFS 1.6.21pre1

Update version strings for the first 1.6.21 prerelease.

Change-Id: I700f0b110373e47f2f471f30ba8eefe9a3b6cf4f
Reviewed-on: https://gerrit.openafs.org/12603
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>