netlink: Move local_address into struct netlink

All the actual netlinks have this, and proper ICMP generation on
point-to-point links is going to need it.

The dummy "null" netlink previously didn't have this parameter; now,
it is mandatory.  This is an incompatible configuration change, but
only for configurations which contain a null netlink - which we think
is only done for testing or debugging.

No other functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>