Update asetkey.8 for KeyFileExt

Update asetkey.8 for KeyFileExt

Prefer KeyFileExt to KeyFile ~everywhere.  Make the main documentation
assume a modern cell with KeyFileExt and rxkad-k5, moving the old
rxkad and KeyFile documentation to a new section,
HISTORICAL COMPATIBILITY.

Note that kaserver is deprecated.

Do not mention the Update Server, which is also disrecommended for
new installations.

Add a copyright statement for the new content.

Change-Id: Idcb4940615a00189b655538a9a190cc35153cc89
Reviewed-on: http://gerrit.openafs.org/11769
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>