(du_files): Don't strip any trailing slash.

(du_files): Don't strip any trailing slash.
Rewrite so that `/' is no longer represented internally as
the empty string.
(count_entry): When appending a file name component,
account for the fact that the current path may end in `/'.
François Pinard reported that `du symlink-to-dir/' was not
equivalent to `du symlink-to-dir/.'.  Now it is.