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.

Use consistent address schema with local and remote machine. Fill
variables with example IPs from my daily test run.

Do not use privileged or NFS source ports for UDP packets as inetd
ignores such packets. This should avoid some sporadic failures.
While there, use variable names consistently in all tests.

Test that IPv6 framents with invalid length or more-fragment bit
drop the whole fragment cache.