ospfd: basic support for VRF NETNS backend

ospfd: basic support for VRF NETNS backend

The change consists in taking into account of the VRF identifier upon
which the ospf socket is created. Moreover, if the VRF is a netns
backend, then it is not necessary to perform the bind operations to vrf
device.
Also, when a VRF instance is enabled, it informs ospf VRF, and automatically
OSPF VRF benefits from it. Reversely, when VRF instance is disabled,
then OSPF VRF will be disabled too.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>