CSS control over translated error pages

Adds %l replacement tag to include CSS file data into an error page.

Adds error_stylesheet option to name a file as the CSS content to insert
into each error pages displayed. (default /etc/squid/errorpage.css)

Adds CSS hooks into the templates.

Adds errorpage.css to squid config directory with current CSS settings
pulled from old templates and demo entries for new hooks.

The combined effect of these is allows company sites to stylize the pages
produced to some extent limited only by the CSS capabilities. Without
worrying about translation texts themselves.

Only works for dynamically translated pages based on updated templates.
Default CSS file may need some cleaning.