Attempt to drop privileges if required.

def DropPrivileges():
  """Attempt to drop privileges if required."""
  if config.CONFIG["Server.username"]:
    try:
      os.setuid(pwd.getpwnam(config.CONFIG["Server.username"]).pw_uid)
    except (KeyError, OSError):
      logging.exception("Unable to switch to user %s",
                        config.CONFIG["Server.username"])
      raise