Switch to scapy with python 3.

Link regress/sys/netinet/frag to build. Get IPv4 and IPv6 fragment
tests into common shape. Add some missing test cases.

Copy IPv6 fragment test cases to IPv4 and adapt to the different
behavior. All tests are run once with pf fragment reassembly and
once with pf disabled and IP input reassembly. Three sub tests are

Copy IPv6 fragment test cases to IPv4 and adapt to the different
behavior. All tests are run once with pf fragment reassembly and
once with pf disabled and IP input reassembly. Three sub tests are
currently disabled as the IP stack behaves differently than pf.

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