Interactive list aircraft display and delete changes

Create two different Time-To-Live parameters for the interactive display
list.

1) Modes.interactive_display_ttl defines for how long the display of an
aircraft persists after the last received message. The default is
MODES_INTERACTIVE_DISPLAY_TTL, which is set to 60 seconds. This replaces
the previous Modes.interactive_ttl/MODES_INTERACTIVE_TTL settings (which
were also 60 seconds)

2) Add an additional Modes.interactive_delete_ttl  parameter, and
default it to  MODES_INTERACTIVE_DELETE_TTL, shich is set to 300
seconds/5 minutes. This defines for how long an aircraft persists in the
list structure after the last received message before being deleted.

Also tidy up a few comments.
Split into separate module files

Ok - this is likely to upset some people. Up until now, the vast
majority of the code has been in just one file - dump1090.c. This file
has grown so that it was approaching of 5000 lines long, and it was
becoming unmanagable. So I've split the file into several modules,
hopefully along fairly logical boundaries. The files are :

1) dump1090.c : Basically just the main() entry function, the help
function, the RTL dongle hardware interface, and a few orphan functions
that  don't really fit anywhere else.

2) mode_s.c : This contains all the mode S / ADSB decoding functions.

3) mode_ac.c : This contains all the mode A & C decoding functions

4) interactive.c : This contains all the functions to maintain an
internal list of aircraft seen over the last period, and functions to
print them out to the local console.

5) net_io.c : This contains all the network input/output functions
allowing data to be passed in/out to/from other receivers, in formats
such as SBS-1/3, Beast, AVR and JavaScript.

Hopefully this should provide an easier way forward if/when more
functions are added.