add basic mpls support to iproute

add basic mpls support to iproute

- Pull in the uapi mpls.h
- Update rtnetlink.h to include the mpls rtnetlink notification multicast group.
- Define AF_MPLS in utils.h if it is not defined from elsewhere
  as is done with AF_DECnet

The address syntax for multiple mpls labels is a complete invention.
When I looked there seemed to be no wide spread convention for talking
about an mpls label stack in text for.  Sometimes people did:
"{ Label1, Label2, Label3 }", sometimes people would do:
"[ label3, label2, label1 ]", and most of the time label
stacks were not explicitly shown at all.

The syntax I wound up using, so it would not have spaces and so it
would visually distinct from other kinds of addresses is.

label1/label2/label3 Where label1 is the label at the top of the label
stack and label3 is the label at the bottom on the label stack.

When there is a single label this matches what seems to be convention
with other tools.  Just print out the numeric value of the mpls label.

The netlink protocol for labels uses the on the wire format for a
label stack. The ttl and traffic class are expected to be 0.  Using
the on the wire format is common and what happens with other address
types. BGP when passing label stacks also uses this technique with the
exception that the ttl byte is not included making each label in a BGP
label stack 3 bytes instead of 4.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>