Move editsitevars page form logic to a backend Perl script.

     www/editsitevars.php3 - The reworked PHP page, including
       a SetSiteVar function bridging to the script via XML.
     backend/{editsitevars,GNUmakefile}.in configure configure.in - New backend script.
     sql/database-fill.sql - Add table_regex 'sitevariables' checking patterns.