IO: Avoid re-binding accepted sockets to VRF

IO: Avoid re-binding accepted sockets to VRF

When VRFs are used, BIRD correctly binds listening (and connecting)
sockets to their VRFs but also re-binds accepted sockets to the same VRF.
This is not needed as the interface bind is inherited in this case, and
indeed this redundant bind causes an -EPERM if BIRD is running as
non-root making BIRD close the connection and reject the peer.

Thanks to Christian Svensson for the original patch and Alexander Zubkov
for suggestions.