[modules] clean up digital cam

Starting to solve #867 and clean digital cam up a bit..

- **replace meter grid by proper distance autoshoot**:
 Previously DC_AUTOSHOOT_DISTANCE would shoot on a 100m UTM north distance grid, seems like nobody actually used that or found this useful.
 So changing this to "proper" distance based autoshoot, where we shoot the next pic  if `dc_autoshoot_distance_interval` is greater than distance to last saved shot position.

Todo:
- [ ] remove `dc_probing` ?
- [ ] choose _one_ coordinate system and harmonize DC_* messages
- [ ] actually test with rotorcrafts