fix sporadic segfault when scanning for hosts

this pull request is intended to fix issue #579.
It basically removes the decoder for host scanning and leverages the "normal" decoder for processing the packets. It also makes sure that only one decoder instance is active at one time while scanning.
> this is something that worries me a lot, what does it happen if the main thread has a different name?

This isn't influenced outside of ettercap. The main thread is being statically created with this name.