The manœuvre node marker should show the ignition orientation prior to ignition for guided burns

[eggrobin]

Following the Frenet frame prior to ignition is useless, even for manual execution, and, now that MechJeb supports Principia burns (MuMech/MechJeb2#1264), it leads to 10 min of confusion as MechJeb desperately attempts to remain aligned to a rotating target (interrupting its timewarp to do so) prior to ignition.