Tracker: add nav_status comments

no functional impact
Tracker: check target of manual control msgs
Tracker: add vehicle struct comments

No functional change
Tracker: remove unused relative_alt
Tracker: move vehicle pos estimate to separate function
Tracker: move bearing and dist calcs to separate function
Tracker: move auto, manual and scan to separate files
Tracker: move pos estimate to separate function
Tracker: move servo control to servos.pde file
Tracker: move startup delay to update_tracking call

This ensure the tracker does not move in any mode until after the
startup delay has passed
Tracker: vehicle state made global
Tracker: auto move servos only with valid vehicle position
Tracker: rename err variable to angle_err

No functional change
Tracker: move nav_status lower in AntennaTracker.pde

No functional change
Tracker: add YAW_RANGE and PITCH_RANGE params

These capture the yaw and pitch range of movement in degrees
Tracker: limit pitch to PITCH_RANGE

Also set servo_limit flags for pitch
Tracker: move servo init to servos.pde
Tracker: remove servo debug
Tracker: reorder servo functions

No functional change.
Flow runs from top to bottom with functions at top of file calling those
lower down.
Tracker: remove outdated comments