Telemetry: add GCSLowDataRateModeEnabled flag

- If GCSLowDataRateModeEnabled is set to YES
  (NO by default) then we only request a
  limited set of telemetry and don't
  request a mission on telemetry connection
Telemetry: no more double negative

- Rename GCSLowDataRateModeEnabled to
  GCSStandardDataRateModeEnabled
Telemetry: clear up comments re GCSStandardDataRateModeEnabled
Merge pull request #99 from fightingwalrus/igcs-170-add-low-datarate-mode

Telemetry: add GCSLowDataRateModeEnabled flag