Update bos addkey/listkeys and KeyFile man pages for asetkey

Update bos addkey/listkeys and KeyFile man pages for asetkey

Clearly prefer asetkey to bos addkey in the KeyFile, bos addkey, and
bos listkeys man pages.  Reference asetkey list and asetkey delete as
alternatives to bos listkeys and bos removekey.  Distinguish between
Authentication Server cells and Kerberos v5 cells and mention the
preferred afs/<cell> principal format.  Add some cautions around
matching enctypes and salts when synchronizing keys with a v5 KDC.

Update man-pages/README for completion of this task, clean up some
other wording, and remove some other now-irrelevant information.

Change-Id: I29b83a61cbdb08de508bdb313524a307e385044b
Reviewed-on: http://gerrit.openafs.org/1938
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>