These functions have long sown confusion among Python developers. Even in
the urllib implementation and tests, they seem to be used in contradictory
ways. A test helper named `sanepathname2url()` has been with us since 2004!

The existing documentation says that these functions deal with URL path
components. But that doesn't fit the evidence on Windows:

    >>> pathname2url(r'C:\foo')
    '///C:/foo'
    >>> pathname2url(r'\\server\share')
    '////server/share'  # or '//server/share' as of quite recently

If these were URL path components, they would imply complete URLs like
`file://///C:/foo` and `file://////server/share`. Clearly this isn't right.

The conclusion I draw is that these functions operate on everything after
the `file:` prefix, which may include an authority section.