Add a manual page for the KeyFileExt file.
Add cross-references from all places which currently reference
KeyFile(5), and update their body text accordingly.
Change-Id: Iab56847fcb59dda0c8a344a626ddb0ff35b98b26
Reviewed-on: http://gerrit.openafs.org/11770
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
