New feature IPv6 active host scan

This pull request introduces a new feature part of the IPv6 enhancements of ettercap:
* introduce a new option to enable IPv6 scan if built with IPv6 support
* new function to probe for active IPv6 hosts on the link
* add new hooks to also listen for ICMPv6 replies or parameter problem
* use the solicited node address to query the LLA if IPv6 address has been defined as targets
* make ip_addr_is_local() reliable when evaluating IPv6 address
* correct parsing of IPv6 address if a host file is being saved or loaded
* some more minor things.

Sorry that this PR is that huge but there was some work to do to get to a state introducing that IPv6 functionality.
the second part will be to fully implement the ND poisoning MITM method. But I didn't want to make this PR bigger.

Cheers.