Changes to prevent stale output in SBS-1 format

The original code calculates Lat/Long only if it receives two DF-17
(subtype 9 or 18) within one second of each other. I have no idea why.
It then caches the results in the aircrafts data structure for use in
the --interactive display.

When SBS-1 style ASCII output is selected (port 30003) the code does not
attempt to calculate Lat/Long from the data just received - instead it
picks it up from the cached information in the aircraft's data
structure.

However, if the data isn't being updated this results in stale Lat/Long
being sent out. This is most likely to occur when the aircraft is at the
extreme edge of the receivers range when it may be getting some DF-17s
containing Lat/Long, but not 2 per second. The program will continue
sending the stale data until the aircraft finally times out (default 60
seconds)

I have modified the code to include a sbsflags variable in the aircraft
data structure. When a new Lat/Long is decoded and put into the
structure, a bit is set to indicate SBS_LAT_LONG_FRESH. Then, once the
Lat/Long is output the first time, the bit is cleared. Thereafter the
code will not populate the Lat/Long fields in the output message until
SBS_LAT_LONG_FRESH is set again.