realname() (former user()) is broken on Windows

[Vlad-Shcherbina]

To reproduce, run the following program:

// [dependencies]
// whoami = "0.8.1"
fn main() {
    whoami::user();
}

or

// whoami = trunk
fn main() {
    whoami::realname();
}

It fails:

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `16`,
 right: `15`

The assertion is here: https://github.com/libcala/whoami/blob/14f0e4a4cebb5debf7284da5b83152689161857c/src/windows.rs#L149

This happens because realname_os() is copy-pasted from username_os(), with GetUserNameW replaced with GetUserNameExW. But unlike GetUserNameW, GetUserNameExW on success returns the number of characters NOT counting the null terminator.