the message being emitted immediately.
Fix computation of reception time so it's more sensible (the block timestamp
is some time after reception of the _end_ of the block, not the start) - this
means that message-emission times are always later than message-reception
times in SBS output, which is a bit more sensible.
Use clock_gettime in preference to ftime.
(except in --net-verbatim mode, where we emit them all)
Move aircraft tracking into track.[ch].
Clean up references to "interactive mode" when tracking
aircraft - we always track aircraft, even in non-interactive
mode.
