AP_Rally: support altitude frames

[IamPete1]

This work is sponsored by Spektreworks.

This adds support for alt frames in rally points. There were some free bits in "flags" that we can use. This assumes that those unused bits are zero. It is possible that they are none zero in storage if uploaded with the old RALLY_POINT MAVLink message.

Tested conversion from old format, all alt frames manually on plane and added new method to the existing TerrainRally plane test.

[WickedShell]

This is strongly desired from a urgent need to get support for absolute or relative rally points supported. A later change that let us have full missions in rally points would be quite interesting, but we'd like to get this in now, and this should be no impact for existing ground stations as we never had assigned a value to those flag bits in the XML. (Out of spec of course is a different story).

[rmackay9]

If the rally point alt frame is set to terrain does a copter actually follow the terrain to the rally point or does it fly a straight line to the absolute altitude (which matches the specified alt-above-terrain)?

[IamPete1]

@rmackay9 Your correct, on copter it flys to the abs altitude and then flys the return based on the RTL_ALT_TYPE parameter. To do it completely we would need a extra variable to know if the returned point is home. If were flying home then we would use RTL_ALT_TYPE to set the alt frame if flying to a rally point we would use the provided alt frame.

I was going to chalk that up as a future improvement, but I could add it to this PR if you prefer.

[rmackay9]

@IamPete1, Ah, OK. Let's not let perfection be the enemy of the good..

Added padding uint8_t unused : 3