path-util: add path_extract_directory(), to match path_extract_filename()

path-util: add path_extract_directory(), to match path_extract_filename()

These two together are a lot like dirname() + basename() but have the
benefit that they return clear errors when one passes a special case
path to them where the extraction doesn't make sense, i.e. "", "/",
"foo", "foo/" and so on.

Sooner or later we should probably port all our uses of
dirname()/basename() over to this, to catch these special cases more
safely.