lib, zebra: Allow for interface deletion when kernel event happens

When zebra gets a callback from the kernel that an interface has
actually been deleted *and* the end users has not configured
the interface, then allow for deletion of the interface from zebra.

This is especially important in a docker environment where containers
and their veth interfaces are treated as ephermeal.  FRR can quickly
have an inordinate amount of interfaces sitting around that are
not in the kernel and we have no way to clean them up either.

My expectation is that this will cause a second order crashes
in upper level protocols, but I am not sure how to catch these
and fix them now ( suggestions welcome ).  There are too many
use patterns and order based events that I cannot know for certain
that we are going to see any at all, until someone sees this problem
as a crash :(  I do not recommend that this be put in the current
stabilization branch and allow this to soak in master for some time
first.

Testing:

sharpd@donna ~/frr4> sudo ip link add vethdj type veth peer name vethjd
sharpd@donna ~/frr4> sudo ip link add vethaa type veth peer name vethab
sharpd@donna ~/frr4> sudo vtysh -c "show int brief"
Interface       Status  VRF             Addresses
---------       ------  ---             ---------
dummy1          down    default
enp0s3          up      default         10.0.2.15/24
enp0s8          up      default         192.168.209.2/24
enp0s9          up      default         192.168.210.2/24
enp0s10         up      default         192.168.212.4/24
lo              up      default         10.22.89.38/32
vethaa          down    default
vethab          down    default
vethdj          down    default
vethjd          down    default
virbr0          up      default         192.168.122.1/24
virbr0-nic      down    default

sharpd@donna ~/frr4> sudo ip link set vethaa up
sharpd@donna ~/frr4> sudo ip link set vethab up
sharpd@donna ~/frr4> sudo ip link del vethdj
sharpd@donna ~/frr4> sudo vtysh -c "show int brief"
Interface       Status  VRF             Addresses
---------       ------  ---             ---------
dummy1          down    default
enp0s3          up      default         10.0.2.15/24
enp0s8          up      default         192.168.209.2/24
enp0s9          up      default         192.168.210.2/24
enp0s10         up      default         192.168.212.4/24
lo              up      default         10.22.89.38/32
vethaa          up      default
vethab          up      default
virbr0          up      default         192.168.122.1/24
virbr0-nic      down    default

sharpd@donna ~/frr4> sudo ip link del vethaa
sharpd@donna ~/frr4> sudo vtysh -c "show int brief"
Interface       Status  VRF             Addresses
---------       ------  ---             ---------
dummy1          down    default
enp0s3          up      default         10.0.2.15/24
enp0s8          up      default         192.168.209.2/24
enp0s9          up      default         192.168.210.2/24
enp0s10         up      default         192.168.212.4/24
lo              up      default         10.22.89.38/32
virbr0          up      default         192.168.122.1/24
virbr0-nic      down    default

sharpd@donna ~/frr4> sudo ip link add vethaa type veth peer name vethab
sharpd@donna ~/frr4> sudo vtysh -c "show int brief"
Interface       Status  VRF             Addresses
---------       ------  ---             ---------
dummy1          down    default
enp0s3          up      default         10.0.2.15/24
enp0s8          up      default         192.168.209.2/24
enp0s9          up      default         192.168.210.2/24
enp0s10         up      default         192.168.212.4/24
lo              up      default         10.22.89.38/32
vethaa          down    default
vethab          down    default
virbr0          up      default         192.168.122.1/24
virbr0-nic      down    default

sharpd@donna ~/frr4> sudo vtysh -c "show run"
Building configuration...

Current configuration:
!
frr version 7.2-dev
frr defaults datacenter
hostname donna.cumulusnetworks.com
log stdout
no ipv6 forwarding
!
ip route 192.168.3.0/24 192.168.209.1
ip route 192.168.4.0/24 blackhole
ip route 192.168.5.0/24 192.168.209.1
ip route 192.168.6.0/24 192.168.209.1
ip route 192.168.7.0/24 99.99.99.99 nexthop-vrf EVA
ip route 192.168.8.0/24 192.168.209.1
ip route 4.5.6.7/32 12.13.14.15
!
interface dummy1
 ip address 12.13.14.15/32
!
interface vethaa
 description FROO
!
line vty
!
end
sharpd@donna ~/frr4> sudo ip link del vethaa
sharpd@donna ~/frr4> sudo vtysh -c "show int brief"
Interface       Status  VRF             Addresses
---------       ------  ---             ---------
dummy1          down    default
enp0s3          up      default         10.0.2.15/24
enp0s8          up      default         192.168.209.2/24
enp0s9          up      default         192.168.210.2/24
enp0s10         up      default         192.168.212.4/24
lo              up      default         10.22.89.38/32
vethaa          down    default
virbr0          up      default         192.168.122.1/24
virbr0-nic      down    default

sharpd@donna ~/frr4> sudo vtysh -c "show run"
Building configuration...

Current configuration:
!
frr version 7.2-dev
frr defaults datacenter
hostname donna.cumulusnetworks.com
log stdout
no ipv6 forwarding
!
ip route 192.168.3.0/24 192.168.209.1
ip route 192.168.4.0/24 blackhole
ip route 192.168.5.0/24 192.168.209.1
ip route 192.168.6.0/24 192.168.209.1
ip route 192.168.7.0/24 99.99.99.99 nexthop-vrf EVA
ip route 192.168.8.0/24 192.168.209.1
ip route 4.5.6.7/32 12.13.14.15
!
interface dummy1
 ip address 12.13.14.15/32
!
interface vethaa
 description FROO
!
line vty
!
end

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>

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