Environment bugfix

This is a more proper fix than the proposed in #1059. @ehabkost argued correctly that the tcpdump thread doesn't really *need* to be in the environment, and it is OK to have it as a thread, just as the screendump thread. By closing the tcpdump thread during postprocess, we won't run into a situation where it is trying to update env['address_cache'] while the environment file is being saved. Moreover, this saves one more active process lying around without necessity.

@Antique, let me know what you think of this. It may be also a good idea to run a large series of tests to see if this effectively solves the problem (it should, IMHO).