lib.lua: Added csum() to compute the IP checksum.

The code includes this important comment:

    NOTE: Checksums should seldom be computed in software. Packets
    carried over hardware ethernet (e.g. 82599) should be checksummed
    in hardware, and packets carried over software ethernet (e.g.
    virtio) should be flagged as not requiring checksum verification.
    So consider it a "code smell" to call this function.
ipv6: Answer ICMP ECHO (ping).
designs.neutronisp: Design to support OpenStack Neutron for ISPs.