C.UTF-8 as default locale

Hi,

Looking through https://wiki.freebsd.org/DevSummit/201810, I noticed the
following entry:

- Make C.UTF-8 the default locale (conrad, dteske(installer))

As this sort of change is better done early, I have put togetger a
simple change introducing C.UTF-8 locale using the same common LC_CTYPE
map as other UTF-8 locales (and it's now the source of symlinks instead
of en_US one), and having all other components use C locale:

https://reviews.freebsd.org/D17833

With that in place, next step is likely to be updating 'default' entry
in /etc/login.conf to include 'charset=UTF-8' and 'lang=C.UTF-8', and
changes to installer are likely to be not needed?