To describe the purpose more accurately, the `master_doc` is now renamed
to `root_doc`.  The old name is still available.  But it is recommeneded
to use new one from now on.

`:download:` role creates duplicated copies when the document contains
two or more the role for the same file, but in different form.  It
considers two paths are different when one contains relative path like
`path/to/../file.dat`.

Internally, `env.relfn2path()` does not normalize the given path in
relative form.  As a result, download role can't detect the same paths
are given.  This adds `os.path.normpath()` to `env.relfn2path()` to
normalize the path.