GH-78079: Fix UNC device path root normalization in pathlib (GH-102003)

We no longer add a root to device paths such as `//./PhysicalDrive0`,
`//?/BootPartition` and `//./c:` while normalizing. We also avoid adding a
root to incomplete UNC share paths, like `//`, `//a` and `//a/`.

Co-authored-by: Eryk Sun <eryksun@gmail.com>