[aprssig] APRS to NMEA

[Robert Bruninga]

APRSdos does it.  Converts all incoming APRS info to 
NMEA GGA on the second serial port for looping to any
Other program that only receives NMEA.

> > Sounds like a useful feature, Scott. My EMA Director 
> > is pretty tied to his mapping program and doesn't want 
> > me to tell him that he has to run an APRS client in 
> > order to see APRS trackers. If we can easily dump 
> > APRS posits in NMEA format to a mapping program, 
> > that'd be great!