lib, zebra: Add new api to specify a label associated with the vrf

lib, zebra: Add new api to specify a label associated with the vrf

For L3VPN's we need to create a label associated with the specified
vrf to be installed into the kernel to allow a pop and lookup
operation.

The new api is:
zclient_send_vrf_label(struct zclient *zclient, vrf_id_t vrf_id,
                       mpls_label_t label);

For the specified vrf_id associate the specified label for
a pop and lookup operation for forwarding.

To setup a POP and Forward use MPLS_LABEL_IMPLICIT_NULL
If the same label is passed in we ignore the call.
If the label is different we update entry.
If the label is MPLS_LABEL_NONE we remove
the entry.

This sets up the api.  Future commits will have the functionality
to actually install into the kernel.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>