Big overhaul of the NIS tutorial, e.g. add examples how to exclude
users (and more changes).

The original patch was
Submitted by: Eric Ogren <eogren@earthlink.net>
PR: 18926
but I received a big
Comment email by: Udo Erdelhoff <ue@nathan.ruhr.de>
where he addressed many nits in Eric's changes and/or suggested how to
rewrite parts in a correct/better way and gave me some further content.

Eric's patches removed some of Bill Swingle's old work. I decided
to keep these but to merge Eric's changes into the existing content.

The result is a very fine NIS tutorial, which is supposed to be extended
by NIS Netgroups content written by Udo Erdelhoff soon (at least he
promised that :-])