[PATCH] [RFC] custom error pages

It has become clear that even with the new langpack setups we need an
area dedicated to local custom ERR_* pages which is integrated with but
fully separate from the bundled translations.

This patch adds a configuration directory /etc/squid/errors.d/ for such
templates to be placed.

To simplify the error page loading a symlink is added from the templated
/usr/share/squid/errors/local to that /etc/squid/errors.d folder and
checked just before loading the errors/templates/ backup.

Also, other ideas on what to call the symlink are welcome. I'm kind of
partial to "local", but if anyone has a better idea we can go with that.

Due to this implementation prioritizing these templates it obsoletes
errors_directory. Docs there are updated to deprecate it for future full
removal.

Amos