AP_NavEKF : Fixes bug in initial earth magnetic field states

The calculation for these states was not being bias corrected
AC_WPNav: rounder corners for short spline segments
AC_WPNav: check acceleration is non zero
AC_WPNav: advance_spline to use dt
AC_WPNav: spline sets origin vel to zero when no prev segment

Issue caught by Michael Oborne
AC_AttControl:  45deg/sec yaw rate for Auto, RTL
AC_WPNav: use atan2f, make methods const
Mission: make get_prev_nav_cmd_index const
AP_Mission: fixed a bug re-entering AUTO

this fixes a bug found by Marco where we would continue in CRUISE mode
with no waypoint if we re-entered AUTO after a mission reset
Rover: added BRAKING_PERCENT parameter

this allows for reverse throttle to be applied for braking in corners
Copter: add drift throttle assist
Copter: Drift throttle assist range increased

Throttle assist is applied from 213 ~ 787
Deadband and constraint added
Copter: add Drift throttle assist constraints
Rover: make log_bitmask 32 bit

this fixes the log before arm
AntennaTracker: fixed mode set using custom mode
AntennaTracker: fixed pitch slew rate limit
Copter: Only play GPS tone when armed
Copter: Remove 80m altitude limit on RTL
Copter: remove do_land's use of RTL_ALT_MAX
AP_Notify: correct dos formatting