Add function for exposing ICMPv6 code

[kouellette]

Add a function for exposing the ICMPv6 code of a ND packet. RFC 4861 requires that certain ICMPv6 types are discarded if the code is non-zero, but currently there is no way to get the code from libndp.

[kouellette]

I've submitted a patch for this with the email subject "[patch libndp] Function for exposing ICMPv6 code"