Send FIN and RST flags for biflows

I want to be able to distinguish flows that terminated properly from the ones that simply timed out or were cut short by the collection window. This implements the client-side of that (sending information if the connection was closed on TCP). As I'd like this to get to the release, this branch contains only the client side and I plan to add the server side later ‒ the server side currently ignores the further flag bits, as can be seen in the current code, so it should not cause any problems:

		v6 = flags & 1
		udp = flags & 2
		in_started = not not (flags & 4)
		out_started = not not (flags & 8)
		if v6:
			size = 16
			tp = socket.AF_INET6
		else:
			size = 4
			tp = socket.AF_INET

I'd like someone to check this makes no difference on the currently used flags.