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.
Functions down the line should be able to fail and return a status.
They can be called either through IPC or directly depending on compile settings,
therefore we need an internal result type for various XR_ERROR_* errors, but also IPC failure.
For now functions in ipc_client_utils.c only return XRT_SUCCESS or XRT_ERROR_IPC_FAILURE.
If there is no IPC failure, the generated protocol will extract and return the called functions'
actual return value from the reply.
v2: make failure results negative
