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?
I think about some distro calling the binary ettercap-gtk (there is one IIRC, fedora maybe).
Moreover one day we will decide to change the binary name (it was ettercap-ng for a while)