handle different AF_INET6 values on different OSes

This pull request addresses build issues on systems other than Linux, because the address family AF_INET6 has different values among the various operating systems.
However, the AF_INET6 values are still necessary to be known on the compiling system for analysing a PCAP traced on a loopback interface also from other systems than Linux.
This reintroduces the value *AF_INET6_DARWIN* which has been taken out in commit d3c9a6a0c1331aa14f42b4497d9a02de2f6d3da8. I think that was because the built failed on Darwin with the same error.
Now it should work.
It also introduces a new AF_INET6 value for Linux, if ettercap is build on an other operating system and want to analyse a PCAP generated on Linux.