AC_AttControl: rename init_targets to relax_bf_rate_controller
Copter: integrate init_targets rename to relax_bf_rate_controller

Also remove unnecessary init_targets from flight mode init functions.
TradHeli: integrate init_targets rename to relax_bf_rate_controller
Copter: flip records starting attitude

Previously flip relied on the attitude controller not updating the earth
frame target during the flip which can't be guaranteed.  Safer for flip
to maintain it's own copy of the original attitude .