zebra: improve interface shutdown behaviour

zebra: improve interface shutdown behaviour

Linux removes IPv6 addresses when the interface is set down. Those
addresses need to be readded when the interface is set up again.

Also, an interface should not be reactivated from shutdown by configuring
an ip address.

Finally, remove the three-state logic for the shutdown setting as its
sole current use may be mild confusion.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>