Add test for port access functions

Tests whether trace_get_source_port and trace_get_destination_port
work correctly.
Fix build bug with UNUSED in libwandio
Added trace_get_fragment_offset to API

New function performs all the bit-shifting, masking,
multiplying and (in the case of IPv6) header skipping necessary to
work out the fragment offset for a packet, in bytes.

Fixed trace_get_source_port and trace_get_destination_port so
that they do not try to lookup port numbers if the packet is
not the first fragment.

IPv6 fragment offsets probably need more testing.
Add new test for fragment offset function

Update test suite to build and run new port and fragment tests.
Return 0 for port numbers for ICMPv6

This brings the behaviour in line with that for IPv4 ICMP.
Merge branch 'getfragoff' into develop