Improved input validation for settings that are acessible by the admin user.

interface/lib/classes/validate_systemuser.inc.php

interface/web/admin/form/server_config.tform.php

interface/web/admin/lib/lang/en_server_config.lng

interface/web/sites/form/ftp_user.tform.php

interface/web/sites/form/shell_user.tform.php

interface/web/sites/ftp_user_edit.php

interface/web/sites/shell_user_edit.php

interface/web/sites/web_vhost_subdomain_edit.php