Trim trailing newline from FormatMessageW

[diaphore]

FormatMessageW always inserts trailing \r\n to system messages which is a minor annoyance when they're fed to Debug but can break formatting with Display.

fn main() {
    use std::env;
    if let Err(err) = env::set_current_dir("???") {
        println!("{:#?}\n{}", err, err);
    }
}

Error {
    repr: Os {
        code: 2,
        message: "The system cannot find the file specified.\r\n"
    }
}
The system cannot find the file specified.
 (os error 2)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (or someone else) soon.

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[alexcrichton]

@bors: r+ 2daa1b7

Thanks!