[PATCH] auto-negotiate error pages

Round 2;

Just the error page auto-negotiation process. This is based on optional
pre-translated error pages supplied by other patch.

This patch:
  * Converts error_directory squid.conf option into an optional override.
    Providing backward compatibility with older configurations and local
customizations.

  * Adds error_default_language to administratively set the backup
language presented by Squid.

  * Fixes design of previously broken hard-coded failover language.
    WAS: default to FreeBSD error install location. Hidden by
error_directory being required.
    NOW: failover to build-time configured errors/ directory 'English'
templates.

  * Adds --enable-auto-locale configure option to enable the following.

  * Adds logic to locate visitors most-preferred of available languages
and present a tailored error page as their reply.

Amos