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

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.
For the User ports (1024 - 49151) we try to keep some sensible balance only
of services that are likely to be found on FreeBSD/UNIX systems. This attempts
to strike a balance between complexity and usefulness.

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.

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

MFC after: 3 weeks (likely to 12-stable only)
Differential Revision: https://reviews.freebsd.org/D23621