Add FILTER_VALIDATE_INCLUDE validation filter for variable includes

There are many security issues that arise from not sanitizing a variable before using it in an include (eg include $script;). The filter extension is intended to prevent this kind of security issues. A validation filter would make it easier and could be the defacto standard when using variable includes. When a static code analyzer is used, it can check if the filter has been used and the variable is safe to be used in include. The options could be "base_path, allowed_streams". The base_path option defines the path where the file should be in. Dots like .. are resolved. Home paths, like ~/foo and ~arnold/ are not allowed (or resolved). Symlinks are not considered. The allowed_streams option would set which streams are allowed. By default none. I feel this is a better option than relying on 'allow_url_include' or RFC: Precise URL include control ( https://wiki.php.net/rfc/allow_url_include). include filter_var($script, FILTER_VALIDATE_INCLUDE, ["base_path" => "path/to/project/", "allowed_streams" => ["phar", "zip"]]); What do you think? Also, does this require an RFC or should I just create a PR? - Arnold