NDP poisoning improved

This pull request intends to bring the NDP poisoner to the same state the ARP poisoner is.
Therefore I introduced new configuration options as equivalents.

In addition, this pull request takes care of the IPv6 packet forwarding. 
However currently only for the OSes Linux, Darwin, BSD and Solaris.
The mechanisms for Darwin, BSD and Solaris have been programmed based on documentation but haven't been tested yet due to lack of available OS for tests. 
So tests are are very appreciated, especially on one of the mentioned OSes.