bgpd: add LP_TYPE_BGP_L3VPN_BIND label type

bgpd: add LP_TYPE_BGP_L3VPN_BIND label type

Redistributing mpls vpn prefixes with a new label
requires picking up unused MPLS local labels.

Today, there is an MPLS label pool which is owned
by the zebra daemon. BGP gets a chunk of labels
that are shared with the multiple usage of the BGP
daemon. A label type attribute is used whenever
BGP needs a new label value.

The 'LP_TYPE_BGP_L3VPN_BIND' label type will be used
to allocate the MPLS labels that will be bound to
the original next-hop, and label value of an L3VPN
update.

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