Update from Denton Gentry with minor bug fixes by me.

Update from Denton Gentry with minor bug fixes by me.
Denny says:
if_sn.c:
        - allocate sc->space using malloc (Scott wanted this).
        - make csr accesses use NIC_PUT/GET, remove sc->sc_csr.
        - add multicast and IFF_PROMISC support. multicast isn't
                well tested, but I made sure the unicast case still
                works.
        - finish the new arp interface, remove sc->sc_enaddr
if_snreg.h:
        - remove struct sonic_reg
if_snvar.h:
        - add NIC_PUT and NIC_GET macros a la if_ae
        - remove sc_txhead, it isn't used.
if_sn_nubus.c:
        - clean up support for my SE/30 card.
        - Move add_nubus_intr from if_sn.c (to keep if_sn.c MI)
if_sn_obio.c:
        - make Q610, C610, Q650, C650, Q800 use EXBUS.
        - PB500 had a "return" where "break" was intended. PB500
                probably wasn't working.
        - move add_nubus_intr from if_sn.c.
        - add an "explanation" why Apple's ethernet addr is encoded
                in token ring format in the PROM.