may have gotten the gremlin; was an interaction that separate used to handle, but...

may have gotten the gremlin; was an interaction that separate used to handle, but in ugly way,
where one cannot tell that the path given to separate was rooted if one just gets a list of
components.  that's why we used to treat the top-level dir as its own component, but that also
isn't very helpful or sensible.  so now separate and join both have a rooted parameter; separate
reports if the path was rooted, and join needs to know that so it can recreate the original
path.