New MITM feature - ND poisoning

This pull requests introduces or finishes the ND poisoning MITM capability of ettercap.
It is based on PR #309, hence can not merged / used separately. Therefore, the ipv6-scan branch has been merged into this branch which makes merging PR #309 not necessary.
It also adjusts some more aspects of ettercap due to limitations to display or handle IPv6 addresses properly (e.g. IP address DNS resolution, GTK connection table, etc...)