FPM: Always use script name in PHP_SELF if cgi.discard_path = 1 and cgi.fix_pathinfo = 0

Description

Currently if PATH_INFO set, then it is used in PHP_SELF if cgi.fix_pathinfo is disabled. This makes sense if the executable is used from PATH_TRANSLATED which is default behaviour. However if cgi.discard_path is enabled, it uses SCRIPT_FILENAME executable instead. This however does not match the documentation which states that PHP_SELF is relative path to the executable because PATH_INFO has nothing to do with executable path.

This was first mentioned in https://bugs.php.net/bug.php?id=68053 and this issue is an extraction of that problem (the primary concern won't be fixed due to a significant BC break). Even though this could be considered as a bug, it is not that critical and it has some BC break concern. It means it should be treated more as a feature change to improve the current behaviour and should go only to master and be mentioned in UPGRADING.