Fix portability issue in the router_xmldir plugin

Use the portable alphasort(3) insead of the versionsort(3) GNU extension.
Return meaningful responses from router_xmldir in case of errors
Include hidden files in router_xmldir directory listing

Hidden files can be filtered out through a XSL stylesheet if needed.
Convert filenames from codeset of the current locale to UTF-8 encoding
Improve router_xmldir output

Root element is now <index> instead of <tree> since the output is not a tree.
Add a path attribute to <index> which contains the content of PATH_INFO.
Only show files and directories and add attributes for their mtime and size.
Fix style in router_xmldir
Add example configuration and XSL stylesheet