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

This is the final half of splitting r358153 in two, in order to avoid a build
system bugs and being able to merge an earlier change to previous releases.

Add a note to UPDATING to avoid people building from very old systems from
having issues with mergemaster

MFC after: 3 days (only 12-stable)

Differential Revision: https://reviews.freebsd.org/D23621