- Split out the {family,socktype}->netid conversion into a separate

- Split out the {family,socktype}->netid conversion into a separate
  function; we now handle unknown protocols more gracefully.

- Cache the return from getnetconfigent() so that we don't have to
  remember to call freenetconfigent() each time. This fixes a memory
  leak that would cause retrying background mount_nfs processes to
  slowly increase their memory usage.