Netlink: MPLS routes in kernel, not fully working yet.

TODO:
- fix static to detect MPLS stack changes on reload
- fix MPLS_ENCAP parsing on route scan

Anyway, Bird is now capable to insert both MPLS routes and MPLS encap
routes into kernel.

It was (among others) needed to define platform-specific AF_MPLS to 28
as this constant has been assigned in the linux kernel.

No support for BSD now, it may be added in the future.