zebra: Layer-2 interface handling

zebra: Layer-2 interface handling

Define interface types of interest and recognize the types. Store layer-2
information (VLAN Id, VNI etc.) for interfaces, process bridge interfaces
and map bridge members to bridge. Display all the additional information
to user (through "show interface").

Note: Only implemented for the netlink interface.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>