Move editexp page form logic to a backend Perl script.

     www/editexp.php3 - The reworked PHP page.
     backend/{editexp,GNUmakefile}.in configure configure.in - New backend script.
     www/experiment_defs.php - Add an Experiment::EditExp class method
                               bridging to the script via XML.
     db/Experiment.pm.in - Add an EditExp worker class method for script arg checking,
                           And a missing description method.
     sql/database-fill.sql - Add to the table_regex 'experiments' checking patterns.