AC_WPNav: add loiter_soften_for_landing method

This resets the position target to the current location.
Copter: soften loiter target when maybe landed

Applies to auto's land, land, loiter, pos hold and rtl flight modes
Copter: add land_complete_maybe flag
Copter: rename land_maybe_complete function
AP_Motors: throttle_pass_through accepts pwm
Copter: move esc calibration to separate file

Restructured into case statement
Replaced use of g.throttle_max with definition
Added more comments
Send message to ground station instead of printing on console (although
probably both are unlikely to be read)
Copter: auto esc calibration

Throttle is raised to maximum and then lowered without any input from
pilot
Copter: add LOG_FROM_STARTUP definition

Uncommenting this line in APM_Config.h will start logging as soon as the
board starts up instead of waiting for the vehicle to be armed
Copter: increase Alt Disparity check to 2m
Copter: cleanup enabling of cli and frsky telem for APM
Copter: low battery msg to severity high
LowPassFilter: add div by zero check
AC_PosControl: 2hz filter on accel error

Replaced hard-coded filter with LowPassFilter class allowing the
filter's to be 2hz on both APM and Pixhawk
AC_PosControl: 4hz filter on z-axis velocity error
Copter: increase autotune limits

Rate D max to 0.020 (was 0.015)
Rate P max to 0.35 (was 0.25)
Stab P max to 20 (was 15)
Copter: use disparity threshold define for pre-arm checks

There are two duplicate checks, one in the pre-arm checks (i.e. checks
run every 15 seconds or so before the vehicle is armed) and one in the
arming checks (run immediately before arming).  The definition in the
pre-arm checks was still using the old hardcoded value.
Compass: always default devid to zero
Copter: allow passthru for ch 9 ~ 14

Based on work by Emile Castelnuovo
Plane: disable the CLI if AFS failsafe enabled on APM2

the flash space is a bit too low for both CLI and AFS
APM_OBC: added heartbeat() method

this is used for when the plane is calibrating sensors, to ensure
heartbeat is continued to the failsafe board