On some platforms and/or some libpcap verisons, libpcap doesn't support a
timeout which makes interactive stop not possible. So we now use a UNIX
signal to break out. Obviously this works only on *nix platforms - which
includes MacOS. For now the problem is not seen on Windows with WinPCAP,
so we should be fine. May need to revisit when we add Npcap support.
Fixes#215, #234
