Switch to scapy with python 3.

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.

Put all make targets in a loop and print a little more what each
IPv6 fragment header test is doing.

Fix tests: Restrict getpid() to lower 16 bit so that it can still
be used as packet id. Now scapy calls nexthopmtu with this name
explicitly in icmp structure.

Use a simpler expression to check the ether type in scapy. This
makes the fragment tests work on FreeBSD. From Ilya Bakulin.

fix typo in fragment payload

Add a test for atomic IPv6 fragment reassembly.