Use FilenameList as type for lists of file names, as used in some
listing-alike APIs. This way we can ensure anything different than just
file names in those lists is rejected outright.
As a consequence, test-big-dirs.pl does not need to prepend the
directory name anymore before calling listing-alike APIs: previously
they didn't fail, but the returned lists contained only invalid
elements (and only their size was checked).
Furthermore, add a new regression test for it.
b5a6f899cd