NMEA stream export uses system locale settings instead of protocol specification

#### Issue details
When Mission Planner exports NMEA data to the external device it uses system locale decimal separator for value strings rendering which may be other than specified in NMEA standard

#### Version
MP 1.3.56

#### Platform
[ X ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
All

#### Hardware type
All

#### Logs
Easily reproducible

No file, test, or entry point is identified. Start by locating the NMEA stream export path and reproduce the output under a locale with a non-period decimal separator. Done means exported NMEA numeric values always use the decimal separator required by the NMEA specification, regardless of system locale.

Written by the indexing model from the issue text.