(#1) Add skeletal handler for MAP to handler error cases

- Add nonce to mapping struct ready for PCP
- Append acted-upon options to response, set reserved fields to
  zero as per RFC
(#1) Switch to sockaddr_storage for mappings

- NAT-PMP code currently passes sockaddr_in about so when using
  memcmp(3) use the size of the passed sockaddr_in, not the size
  of the sockaddr_storage in the mapping entry
(#1) IPv6 client addresses are also currently unsupported

- Zero out more of the reserved bytes of response packets
(#1) Initial attempt at MAP support

- Add {from,to}_sockaddr for converting between IPv6 address
  (containing either real IPv6 address or IPv4-mapped on) and
  port to the appropriate sockaddr and vice versa