Abandons the assumption that in oxr_system.xdevs[], index 0 is HMD,
1 is left controller, 2 is right controller.
Now to represent the dynamically assigned roles, oxr_system.role contains
the index for a device in oxr_system.xdevs[] for head, left and right.
This role assignment happens on the client side and currently can not be updated
from the server side.
Also adds an enum that device drivers set indicating allowed assignments
(many controllers are physically designed to be held in a specific hand).
This also adds support for configurations with only a HMD and a right controller.
