This a big patch, which now mostly finishes the separation work
of isdn_net_dev, isdn_net_local and struct net_device.
The latter two are allocated per network-layer known network interface,
while isdn_net_dev is the entity which is accessed using isdnctrl,
i.e. a per-channel thing.

Since we allow for channel bundling, isdn_net_local, the priv data of
an ISDN network interface, gets a list of isdn_net_dev's which can be
used for transfering data on that interface.