This is related to Task T34861 to increase up & track axis options for TrackTo actuator. I've just added it to differential to facilitate an easier review.
With the patch applied you can select X, Y and Z axis for the Up axis, and X, Y, Z, -X, -Y and -Z for the track axis.
Related to the implementation I have used the algorithm from Trackto constrain placed in constrain.c but adapted to be used with MOTO library.
The wiki docs are here (http://wiki.blender.org/index.php/User:Lordloki/Doc:2.6/Manual/Game_Engine/Logic/Actuators/Edit_Object#Trackto_Actuator).
Test file is here: {F97623}
I have also uploaded 2 screenshots showing the UI modifications to the TrackTo actuator:
{F91992} {F91990}
Reviewers: moguri, dfelinto
Reviewed By: moguri
CC: Genome36
Differential Revision: https://developer.blender.org/D565
1bf87fa26c