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Tracker: remove outdated comments
Tracker: limit yaw to YAW_RANGE
Tracker: add read_radio