(FILESYSTEM_PREFIX_LEN, PARAMS, ISSLASH): Remove; now in dirname.h.

(FILESYSTEM_PREFIX_LEN, PARAMS, ISSLASH): Remove; now in dirname.h.
Do not include <stdio.h>, <assert.h>; no longer needed.
Include <string.h>, <dirname.h>.
(base_name): Allow file names ending in slashes, other than names
that are all slashes.  In this case, return the basename followed
by the slashes.  This is more general, and can be used in places
where the original base_name purposely had an assertion failure.
(base_len): New function.