zebra: lingering IP address after deletion (BZ#486)

zebra: lingering IP address after deletion (BZ#486)

zebra address bookkeeping is a mess. this is just a workaround to have
IPv4 address deletion somewhat working on Linux.

the if_unset_prefix call is synchronous, when it returns success the
address deletion completed successfully. this is either signaled by a
netlink ACK or by an OK return value from ioctl().

This version is wrapped by #ifdef HAVE_NETLINK so we don't touch the
BSDs for now.

* zebra/interface.c: On Linux, update zebra internal state after
  deleting an address.

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