Cleanups, Crash fixes, IPv6 Support, IPv4/IPv6 non-TCP/UDP dumping, ...

The following pull request contains different changes which were necessary to use pkt2flow in our debug setups. It starts with different changes which were necessary to get it compiling cleanly and is followed by different fixes, the possibility to dump non-TCP/UDP IP traffic from one host to another and ends with the incremental introduction of the IPv6 support. There are some fixes here and there which aren't put in front because I've noticed the bugs too late and didn't want to rebase everything again.

Cc: @lindnermarek
Cc: @simonwunderlich