Set link state correctly in lwIP for interfaces that support
the NETOPT_LINK option. Interfaces that do not support it
(like tap for native arch) remain up all the time.
Netdevs that support NETOPT_LINK but do not send NETDEV_EVENT_LINK_UP/DOWN
events will end up with a mismatched link state - but DHCP would
already not start for them either.
Use netifapi to signal lwIP to do the work in its own thread.
60d5d1aa74