[PATCH] Error Page code upgrades

Languages: Error page auto-negotiation

The error pages component has finally passed testing with good results.
This patch bundles the entire code set to be updated.

* Enables default-on but optional 'make dist' translation of .po files
into error page templates. This adds dependency on po2html tool.

* Installs and uninstalls translated templates properly if present.
Absence of any dynamic translation files is non-fatal.

* Upgrades the ErrorState object from a dumb data-carrier to a class
handling several aspects of its own content.

* The major aspect of the above is locating and producing output
based on the request accept-language specs. This allows automatic
localization of the message to the user/visitors own language.

* Changes the error_directory squid.conf setting to a fully
optional value, and to act as a per-squid override of the
language localization. So as to remain backwards-compatible
with any existing error customizations.

* Currently requires --enable-auto-locale to enable actual
auto-negotiation sequences on each error result. This will remain
optional until enough translations have been completed to consider
turning this behavior on useful.

* Adds the error_default_language setting to squid.conf
which changes the basic default language from English to
some local variant. But only if auto-selection fails.

* Legacy blanket translations shipped with squid are now to be
deprecated in favor of ISO 639 and ISO 3166 coded versions as
the translations arrive.

Amos

-- 
Please use Squid 2.7.STABLE3 or 3.0.STABLE8