Fix cork_path_dirname near root of filesystem

There were a couple of bugs in the results of the `cork_path_dirname`
function when applied to the root directory, or to the root directory's
immediately children.
cork_path_cwd

This patch add a function for retrieving the current working directory
as a `cork_path` instance.  There is also a function for overwriting the
contents of an existing `cork_path` with the current working directory.
Merge branch 'feature/cwd' into develop

* feature/cwd:
  cork_path_cwd