Copter: simple avoidance (i.e. stopping) should work in RTL, Auto, Guided, etc

Copter's object avoidance only works in Loiter, Guided mode's velocity control and AltHold. We should extend it to work in more modes including RTL, Auto and Guided (Waypoint).

No files or tests are named. Start by tracing Copter object avoidance in the existing Loiter, Guided velocity-control, and AltHold paths, then compare the RTL, Auto, and Guided waypoint mode handling. Done means simple avoidance, such as stopping, also works in the requested modes.

Written by the indexing model from the issue text.