Implement Mode A/C decoding

First attempt at decoding legacy SSR Modes A and C.

If the command line switch --modeac is used, the program will now
attempt to recover Mode A/C signals contained in the raw I/Q data
stream. The current recovery mechanism is quite strict and does not cope
well with overlapping and corrupt SSR replies. I estimate that less than
20% of possible returns are decoded correctly. Hopefully over the next
few iterations this can be improved.

If outputting raw data it is recommended to use the --net-ro-size and
--net-ro-rate command line options to reduce the number of very small
ethernet packets that will be generated by mode A/C replies.