map: "set home(altitude)" breake RTL

To reproduce:
1: take off with AC, in map, right-click and set home(NO altitude), or try RTL after moving the copter.
2: observe that RTL works as expected.
3: right click in map, select set home WITH altitude.
4: observe that RTL will just land in spot, - even is set with no altitude later on, the AC will only land , not RTL anymore.
- this is likely to be an issue with MAVProxy *and* ArduCopter

No files or tests are named. Start by reproducing the map right-click “set home” workflow with and without altitude, then trace the MAVProxy and ArduCopter boundary involved in RTL. Done means RTL continues to return to home after setting and later clearing the home altitude.

Written by the indexing model from the issue text.