[modules] dc: 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, storing the last shot position and
shoting the next one if dc_autoshoot_distance_interval is greater than distance to last saved shot position.

Some more cleanup:
- remove unused dc_probing
- remove unused DC_IMAGE_BUFFER_TPI
[modules][messages] send pos as LLA in DC_SHOT and DC_INFO
[modules] dc: workaround to use waypoints in fixedwing and rotorcraft firmware
[modules] digital_cam_x: add dc.h header
[conf] example to test rotorcraft cam and dc stuff
[modules] dc: more cleanup

- remove unneeded dc_buffer* variables
- replace SENSOR_SYNC_SEND with DC_SHOT_SYNC_SEND which defaults to TRUE
- add dc_distance function
- use dc_gps_count in distance mode
- cleanup some variables
[modules] digital_cam: add heading and hmsl in DC_SHOT message

also fix DC_SHOT in hackhd
Merge pull request #936 from paparazzi/digital_cam_cleanup

[modules] clean up digital cam

- actually use position in DC_SHOT message to display marker in GCS
- works for rotorcrafts as well now
- DC_SHOT message now with lat/lon/alt/hmsl instead of UTM and psi(heading) additionally to course
- general cleanup
- **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.