[recovery-netstack] Send ICMPv6 in response to exceeded MTU

- Add the ability to send ICMPv6 "packet too big" messages in
  response to attempting to forward a packet over a link with
  an MTU too small for the packet
- Store MTUs as u32s rather than usizes (ICMPv6 "packet too big"
  messages encode the MTU in a 32-bit field)

Test: None; we don't have IPv6 supported for test dummies yet;
      once we do, we will add tests for this and other ICMPv6
      functionality

Change-Id: I42b24f8fedbce3c97ab6376e86da949a08ed2dff