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

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

Include <string.h> instead of <stdlib.h>.
Do not include <assert.h>; no longer needed.
Include xalloc.h.
(memrchr): Remove decl.
(dir_name_r): Remove.
(dir_len): Renamed from dirlen.  All callers changed.
Rewrite in terms of base_name, for simplicity and consistency.
(dir_name): Never return NULL.  All callers changed.
Do not include <stdlib.h> in test program; no longer needed.
return 0; is fine for test program.