make openafs 1.6.4

The wrong commit was tagged for the 1.6.3 release by accident.
Update the version strings to 1.6.4, and adapt the release notes.

Change-Id: I0c4f8e796a1017aebaaf6c219fd12e1547ad9319
Reviewed-on: http://gerrit.openafs.org/9983
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>