dhcp6: Sanitize DHCPv6 IA option parsing

Sanitize code for parsing DHCPv6 IA NA and TA options and their
nested Status options so that the options can be fully and
properly ignored should they not be conformant to the specification.

Do this by defining a proper DHCP6Option structure and sending that
structure to the parsing function. The parsing function will then
not manipulate either any option data pointers or their lengths in
order to iterate over the current option. Needless to say, this
affects a few files including the test program.