Make arl(4) MPSAFE:

Make arl(4) MPSAFE:
- Add a mutex to the softc and use it to protect the softc and device
  hardware.
- Setup interrupt handler after ether_ifattach().
- Use a private timer instead of if_timer/if_watchdog.
- Retire arl_unit from the softc and use if_printf() and device_printf()
  instead.

Note that the unpatched driver in 6.x and later does not work with the
hardware, so the one person who had volunteered to test the patch wasn't
able to test it.