Moved validation logic from Voyager driver to Search controller -- this should really apply to ALL drivers.

Added file_exists check before deleting session file in destroy() of VuFind/Session/File.php to prevent Warning: unlink(): No such file or directory

Prevent adding a module if it's already in the list.
Because on some systems it's not possible to define environment variables, you should be able to add the module to the normal module list (defined at the top of this file). Problem occures when you work with different systems for dev/test/prod etc. and want to share the same code base.