Updated chapter 2, sections 1-3 of the Admin Guide

Updated chapter 2, sections 1-3 of the Admin Guide

Replaced some references to the Authentication Database with Kerberos.
Removed text about obsolete tools like rcp, inetd, and rlogin.
Corrected references to AFS Product support by replacing them with links
to the OpenAFS Support page. Added warnings about using the wrong fsck binary
with inode and namei-based fileserver binaries. Removed an obsolete paragraph
about ThisCell and how it interacts with the Authentication Database.

LICENSE BSD
FIXES 124931

Reviewed-on: http://gerrit.openafs.org/10
Verified-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Verified-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Russ Allbery <rra@stanford.edu>