F3.12 GPS status: fix, accuracy, sky view and raw stream #331

Scope: the F3.12 row in docs/scope.md (#330). The spec comes first (docs/features/F3.12-….md).

This is the GPS status for a GPS source (F3.6, #205):

• Fix: position, altitude, speed and course, fix type, and time.
• Accuracy: the estimated error in metres where the receiver or gpsd gives it. Otherwise the DOPs and the number of satellites used.
• Sky view: a polar diagram of the satellites in view, by azimuth and elevation, with signal strength, used or not, and constellation.
• Raw stream: the NMEA lines from the serial port or TCP, or gpsd's JSON, in the Console panel (F1.28).

It depends on two APRS.NET gaps, both extended for this:

• G17 (NMEA, Capability: GPS positions from NMEA and gpsd APRS.NET#203): GSV, GST, and the DOPs from GGA and GSA.
• G30 (gpsd, gpsd client APRS.NET#127): TPV error estimates, SKY reports, and the raw lines read.

It's built after F3.6.