Adjust --interactive display

Allow 8 character Longitudes to cope with -179.999

Change the type of signalLevel to unsigned chae. Negative signal levels
make no sense.
Change the message length decision

Assume that DF0-DF15 are all short 56 bit messages
Assume that DF16-DF31 are all long 112 bit messages

This helps us error detect and error recover for bits in the first byte