/etc/services: attempt to bring the database to this century 1/2.

This is the result of splitting r358153 in two, in order to avoid a build
system bug and being able to merge the change to previous releases..

Document better this file, updating the URL to the IANA registry and closely
match the official services.

For system ports (0 to 1023) we now try to follow the registry closely, noting
some historical differences where applicable.

As a side effect: drop references to unofficial Kerberos IV which was EOL'ed
on Oct 2006[1]. While it is conceivable some people may still use it in some
very old FreeBSD machines that can't be replaced easily, the use of it is
considered a security risk. Also drop the unofficial netatalk, which we
supported long ago in the kernel but was dropped long ago.

Leave for now smtps, even though it conflicts with IANA's submissions.
The change should have very little visibility, if any, but should be a
step closer to the current IANA database.

[1] https://web.mit.edu/kerberos/krb4-end-of-life.html

MFC after: 2 weeks