zebra: add ZEBRA_IFC_QUEUED to keep track of kernel state

zebra: add ZEBRA_IFC_QUEUED to keep track of kernel state

As there are timeframes when we don't get a notification from the kernel
about new addresses. (e.g. while Linux performs IPv6 DAD), we need to
have some information whether an address has been sent to the kernel or
not.

One case where this is relevant would be a user adding an IPv6 address,
but deleting it before DAD has been complete. With the next patch which
removes some (ill assuming) synchronous parts in address setup,
ipv6_address_uninstall would not know whether or not it has to actually
delete the prefix from the kernel. Resolving these windows where we lack
information is what the flag ZEBRA_IFC_QUEUED is intended for.

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