Copter: add stick mixing for height in Auto and RTL

[be-ska]

I know this is a very old issue #11251, but I think this functionality is still very handy, in particular for agricolture missions and RTL. My solution is pretty straightforward: after input shaping the pilot input, I just add some offset in AC_WPNav to:

target_pos_neu_m.z
target_vel_neu_ms.z
target_accel_neu_mss.z

This was tested and used extensively with 4.5.7, I've just changed some bits to comply with 4.6.x, in particular I see everything is in meters now lol. This PR was only tested in SITL, where everything seems to work properly.

To test this:

1. launch SITL
2. set AUTO_OPTIONS and/or RTL_OPTIONS 3rd bit

param bitmask set AUTO_OPTIONS 3 
param bitmask set RTL_OPTIONS 3

3. fly in AUTO or RTL and just apply some throttle input. You shouldn't be able to crash, because when approaching LAND_ALT_LOW the down speed is limited to LAND_SPEED and at 3 m (hardcoded) to 0.