bgpd: add 'mpls bgp l3vpn-multi-domain-switching' command

When acting as intermediate device for BGP signaling, and
as transit device for data traffic, the device is not able
to modify the label value from incoming MPLS VPN updates:

- as BGP device, modifying the label value is necessary
when redistributing VPN prefixes with its own next-hop.
- as transit device that connects two ethernet segments
on separate interfaces, the return MPLS traffic must be
handled: the modified label value must be swapped with
the original label value and sent back to the original
next-hop.

The border router use case can be taken as example, when
it acts both as transit and as BGP device:
- When receiving updates from a border router peer, and where
interior traffic is expected to transit through the local
border router.
- When receiving updates from interior devices, and where
exterior traffic will transit through the local border router.

In those two situations, a new label is bound to the received
entry, and the entry is advertised to a new peer with the new
label. In the same time, an MPLS entry is created to handle
return traffic with the new mpls label: the traffic would be
swapped to the original MPLS label and the original next-hop.

This is the first commit of a series of patches, that address
the above mentioned issue.
The first commit introduces a new per-interface command:

> interface eth0
>  [no] mpls bgp l3vpn-multi-domain-switching
> exit

This command will authorise mpls vpn updates to have a new
label value bound to the mpls vpn routes received over that
interface.

Link: https://www.rfc-editor.org/rfc/rfc3107.html#section-3

> When a BGP speaker redistributes a route, the label(s) assigned to
> that route must not be changed (except by omission), unless the
> speaker changes the value of the Next Hop attribute of the route.

Link: https://www.rfc-editor.org/rfc/rfc3031.html#section-4.6
Link: https://www.rfc-editor.org/rfc/rfc4364.html#section-10
sub-chapter b.

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

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