Return correct autopilot custom mode

- Take into account custom modes
  based on MAV_AUTOPILOT and MAV_TYPE
  since fixed wing and quad custom
  modes differ. Will need to expand
  to all MAV supported types and
  autopilots down the road.
Use static NSDictionaries for custom_mode lookups

- Will need these lists for UI eventually but I
  don't want to get ahead of myself just yet.
Probably closer to PR feedback
Merge branch 'fix-alt-hold-status'