Rework DNS resolution

As announced I took on the task to rework the DNS resolution and to make it threaded in background.
Comments are welcome. As soon as  everybody is fine with it I'll rebase the commits before merging.
Well if ettercap want's to resolve a IP address that is in the hosts list and which is also defined in /etc/hosts, `getnameinfo()` resolves it from the local file and caches it in ettercap's resolve cache. If the name in the local file changes, ettercap does not reresolve it again because it's already in the cache.

I can not think of a use case that would benefit from leaving local file name entries volatile.