Many functions returned void and were assumed to always succeed, and some functions
returned only a bool to indicate vague success/failure.

Now that these functions get piped over IPC all of them have to be able to indicate
an IPC failure like for example an unreachable service.

With the xrt_result_t return type they now have the opportunity to report various
types of failures.

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

Instead of maintaining this chunk of code and build options, the Vulkan
loader can be used to load up validation. This has the advantage that no
layer name needs to be hard coded inside Monado, which was subject of
change recently.

Instead of using our own environment variable we can easily set the one
from the loader, e.g. `VK_INSTANCE_LAYERS=VK_LAYER_KHRONOS_validation`.