Only normalize the cached user.dir property once

JDK-8198997 introduced normalization for the cached "user.dir" property in order to avoid "inefficient, repeated normalization".

However WinNTFileSystem.getUserPath(), the only
place where the cached and normalized "userDir" value is used, still calls normalize() on each invocation:

class WinNTFileSystem extends FileSystem {
...
    private final String userDir;
...
    public WinNTFileSystem() {
...
        userDir = normalize(props.getProperty("user.dir"));
  }
...
    private String getUserPath() {
...
        return normalize(userDir);
    }

This is not necessary and can be improved.