Shift a name from PATH_INFO to SCRIPT_NAME, returning it
If there are no remaining path segments in PATH_INFO, return None.
Note: 'environ' is modified in-place; use a copy if you need to keep
the original PATH_INFO or SCRIPT_NAME.
Note: when PATH_INFO is just a '/', this returns '' and appends a trailing
'/' to SCRIPT_NAME, even though empty path segments are normally ignored,
and SCRIPT_NAME doesn't normally end in a '/'. This is intentional
behavior, to ensure that an application can tell the difference between
'/x' and '/x/' when traversing to objects.