T: macpdu2wireshark: gentle exit (and some other minor work in the T tracer)

When outputting to a file, typing ctrl+c exits the program without calling fclose(), leading to loss of traces in the output file.

Let's intercept ctrl+c and exit gently, calling fclose().

Also include a small fix for compilation of _check_vcd, some work on extract and hacks/plot-ofdm, minor stuff.