bgp: ipv4 unicast NRLI sanity checking

[taspelund]

Add sanity checks for the contents of IPv6 Unicast NLRI, i.e. making sure they only contain valid unicast addresses.

Ranges to drop as invalid:

IPv4:

• 127.0.0.0/8
• 224.0.0.0/4
• 240.0.0.0/4
• 169.254.0.0/16 (optional, as this range is sometimes used as loopback ranges similar to RFC 1918)

[rcgoodfellow]

One more check would be to make sure we don't get an NLRI whose next-hop is itself.
e.g.
10.0.0.1/32 via 10.0.0.1

[rcgoodfellow]

Moved IPv6 part of this ticket to #211