zebra: Start of work to get data about mpls from kernel

zebra: Start of work to get data about mpls from kernel

a) We'll need to pass the info up via some dataplane control method
(This way bsd and linux can both be zebra agnostic of each other)

b) We'll need to modify `struct interface *` to track this data
and when it changes to notify upper level protocols about it.

c) Work is needed to dump the entire mpls state at the start
so we can gather interface state.  This should be done
after interface data gathering from the kernel.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
Signed-off-by: Mark Stapp <mstapp@nvidia.com>