Fix the pkt_info -> NO_PI param to the C ext function

So that it is easier to reason about when calling from ruby land. Add comment
in C land that explains how the param maps to the bit in the ifreq struct.
Fix the get/set hwaddr codepath

C ext was incorrectly handling the setting (and getting) of the hwaddr field
for the interface(s).
Shorten exception message for incorrect ip address