Fix pg_rewind which can be run as root user

Hi all,

I was just going through pg_rewind's code, and noticed the following
pearl:
    /*
     * Don't allow pg_rewind to be run as root, to avoid overwriting the
     * ownership of files in the data directory. We need only check for root
     * -- any other user won't have sufficient permissions to modify files in
     * the data directory.
     */
#ifndef WIN32
    if (geteuid() == 0)
    {
        fprintf(stderr, _("cannot be executed by \"root\"\n"));
        fprintf(stderr, _("You must run %s as the PostgreSQL superuser.\n"),
                progname);
    }
#endif

While that's nice to inform the user about the problem, that actually
does not prevent pg_rewind to run as root.  Attached is a patch, which
needs a back-patch down to 9.5.

Thanks,
--
Michael