std::path::Path prefix parsing in Windows is inconsistent with other OS's

I tried this code:

```rust
Path("//tmp/abs_evil2.txt").components()
```

I expected to see this happen: 

```
RootDir, RootDir, Normal("tmp"), Normal("abs_evil2.txt")
```

Instead, this happened: 

```
Prefix(PrefixComponent { raw: "//tmp/abs_evil2.txt", parsed: UNC("tmp", "abs_evil2.txt") })
```