MacOS Classic catdir() rewrite from Thomas Wegner

MacOS Classic catdir() rewrite from Thomas Wegner
(backward incompatibility, but a deliberate one,
the old version simply is broken in its logic),
also documentation updates, and as suggested replicated
the File::Spec::Unix documentation updates also on the
File::Spec documentation.

TODO: there seems to be duplication of documentation
between File::Spec and File::Spec::Unix.  I think
the ::Unix should be left only with specific UNIXisms,
and all the generic documentation should be in ::Spec.

p4raw-id: //depot/perl@12440