BGP: Unnumbered peering in a VRF

BGP: Unnumbered peering in a VRF

Code changes to make unnumbered peering work in a VRF. The changes have
to do with locating the interface in the correct VRF (in order to look for
neighbor address) in the case of outgoing connections and when specifying
source address as well as fetching the correct instance for an incoming
connection based on reading the device the socket is bound to (the multi-vrf
socket option in the kernel).

Additionally, for IPv4 unnumbered peering in a VRF (based on /30 or /31
addresses), bind to the VRF rather than the interface.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-9311
Reviewed By: CCR-4192
Testing Done: Manual, bgpsmoke