We can't use a positional index on an original string that we got from
its lower-cased equivalent. Implement our own IndexFold() function b/c
the std lib does not have one.
* matcher: Add `split_path` option to file matcher; used in php_fastcgi
* matcher: Skip try_files split if not the final part of the filename
* matcher: Add MatchFile tests
* matcher: Clarify SplitPath godoc