IPv6 support for dns_spoof plugin

Updates dns_spoof plugin adding the capablilty to spoof also for IPv6 address:
* AAAA records
* handle PTR queries for IPv6 reverse pointers
* able to provide AAAA answer for MX queries

Additionally it fixes an issue with the MX records which only allowed a domain length of 8 chars (as in the example).

The etter.dns file has been amended with new examples.

The issue of ICMP error re-responses has been fixed by dropping the query which has been spoofed.