owl_mac_bytes_to_string() now uses a static buffer, so the caller do not
need to free the returned buffer manually. Because of this static
buffer, it is not thread-safe anymore, therefore we introduce
owl_mac_bytes_to_string_r() that is a thread-safe version.
In capture(), the pcap_loop() call was blocking, so the program did not
quit if no packet came on rtap_iface. This is now fixed by a call to
pcap_breakloop() in the signal handlers.