Error page auto-negotiation.

Error page auto-negotiation.

  * 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.