Use is_sep_byte, instead of is_verbatim_sep when parsing Prefix::DeviceNS

[CDirkx]

Currently on Windows, when parsing the prefix of a path in the device namespace, e.g. \\.\C:\foo.txt, only \ is recognized as a separator. This means that for the path \\.\C:/foo.txt, the entire path is parsed as the prefix, instead of only \\.\C:. This is also inconsistent because / is recognized as a separator in the rest of the path, e.g. \\.\C:\foo/bar.txt, just not in the prefix.

The documentation mentions no such restriction on the prefix, and prefixes delimited by / are handled fine on Windows:

use std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut file = File::create(r"\\.\C:/foo.txt")?;
    write!(file, "12345")?;
    drop(file);

    let mut file = File::open(r"C:\foo.txt")?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;

    assert_eq!("12345", contents);

    Ok(())
}

Therefore it is my belief that the current behavior is a bug, which this PR addresses.

[rust-highfive]

r? @kennytm

(rust_highfive has picked a reviewer for you, use r? to override)

[CDirkx]

I'm closing this PR in favor of #78833