fixed tcpdumping (option -M)

Fix for issue #68

* added checks to ensure provided directory for dump files is valid and correctly formatted, defaults to CWD if not provided
*  fixed possible crash in case dump thread is canceled and cleanup function is called twice
* fixed possible crash in case dump thread is canceled and remove_flow() is called before dump thread actually terminated
* fixed that uninit_flow() invalidates all existing pointers to flows following the one in question