(path_concat): Rewrite to return new parameter.

(path_concat): Rewrite to return new parameter.
(do_copy): Update uses of path_concat to use new parameter.
Use that new pointer to compute correct offset for make_path_private.
Before, cp --recursive --parents SRC DEST failed when SRC was
an absolute file name.  E.g.,
% cd /tmp; rm -rf d f; mkdir d; touch f; cp -PR /tmp/f d
cp: tmp: No such file or directory
Reported by Horst von Brand vonbrand@sleipnir.valparaiso.cl.