bgpd: new vpn-policy CLI

bgpd: new vpn-policy CLI

PR #1739 added code to leak routes between (default VRF) VPN safi and unicast RIBs in any VRF. That set of changes included temporary CLI including vpn-policy blocks to specify RD/RT/label/&c. After considerable discussion, we arrived at a consensus CLI shown below.

The code of this PR implements the vpn-specific parts of this syntax:

router bgp <as> [vrf <FOO>]
    address-family <afi> unicast
        rd (vpn|evpn) export (AS:NN | IP:nn)
        label (vpn|evpn) export (0..1048575)
        rt (vpn|evpn) (import|export|both) RTLIST...
        nexthop vpn (import|export) (A.B.C.D | X:X::X:X)
        route-map (vpn|evpn|vrf NAME) (import|export) MAP

        [no] import|export [vpn|evpn|evpn8]
        [no] import|export vrf NAME

User documentation of the vpn-specific parts of the above syntax is in PR #1937

Signed-off-by: G. Paul Ziemba <paulz@labn.net>