Tri: _min_throttle interpreted as 0~1000 range for throttle_lower flag

limit.throttle_lower flag becomes true when the throttle passed into the
motors lib (which is in the 0 ~ 1000 range) is below _min throttle.
This makes the interpretation of the THR_MIN parameter consistent
between the main code (which uses 0 ~ 1000 range) and the motors lib
(which previously used the RC3_MIN ~ RC3_MAX range).
The remaining problem however is that the output of the motors continues
to use THR_MIN as if it were a pwm.  I don't believe this is a dangerous
problem however.
MotorsMatrix: _min_throttle interpreted as 0 ~ 1000 range for throttle_lower flag

Also trigger throttle_upper flag when throttle in reaches 1000
SingleCopter: set throttle upper and lower flags
CoaxCopter: set throttle upper and lower flags
AC_AttControl: remove some old comments
AC_PosControl: remove completed to-do comments
Copter: ReleaseNotes for AC3.2-rc11
HAL_Linux: support direct UDP output from UART drivers

this allows safe operation over WiFi links without MAVProxy
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