Reduce overhead of normalizing file paths

UnixFileSystem.normalize can be simplified to this:

    public String normalize(String pathname) {
        int doubleSlash = pathname.indexOf("//");
        if (doubleSlash >= 0) {
            return normalize(pathname, doubleSlash);
        }
        if (pathname.endsWith("/")) {
            return normalize(pathname, pathname.length() - 1);
        }
        return pathname;
    }

.. which is more efficient overall, especially so during startup/warmup (to the tune of a couple of us/op on new File(path)).