extdev: add support for external device handlers

extdev: add support for external device handlers

This allows to integrate external daemons that configure network devices with
netifd. At startup, netifd generates device handler stubs from descriptions in
/lib/netifd/extdev-config via the mechanism in handler.c. These are then added
to the list of device handlers. Device handlers stubs act as relays forwarding
calls against the device handler interface to the external daemon.

Signed-off-by: Arne Kappen <arne.kappen@hhi.fraunhofer.de>