Fix internal link->type vs. hwaddr.type handling

I'd supersede pull #117 with this one, without to change hwaddr.type to hwaddr.arp_type and
without to intrduce to add __ni_iftype_2_arp_type mapping which does not work again:

ni_objectmodel_netif_get_hwaddr puts only the address data without the type to the wire and ni_objectmodel_netif_set_hwaddr sets only the address data now. The netif->hwaddr dbus
property needs cleanup as already stated in the ni_objectmodel_netif_properties.

The hwaddr type is often unknown (void) on the client side (we should not require to have it
set correctly in the config or try to guess), the server knows the type and has to verify it in
setter methods like it is done for the wireless access_point now.