GH-127236: `pathname2url()`: generate RFC 1738 URL for absolute POSIX path (#127194)

When handed an absolute Windows path such as `C:\foo` or `//server/share`,
the `urllib.request.pathname2url()` function returns a URL with an
authority section, such as `///C:/foo` or `//server/share` (or before
GH-126205, `////server/share`). Only the `file:` prefix is omitted.

But when handed an absolute POSIX path such as `/etc/hosts`, or a Windows
path of the same form (rooted but lacking a drive), the function returns a
URL without an authority section, such as `/etc/hosts`.

This patch corrects the discrepancy by adding a `//` prefix before
drive-less, rooted paths when generating URLs.