This is the basic code enabling alternate control of tasks between the
regular kernel and an embedded co-kernel. The changes cover the
following aspects:

- extend the per-thread information block with a private area usable
  by the co-kernel for storing additional state information

- provide the API enabling a scheduler exchange mechanism, so that
  tasks can run under the control of either kernel alternatively. This
  includes a service to move the current task to the head domain under
  the control of the co-kernel, and the converse service to re-enter
  the root domain once the co-kernel has released such task.

- ensure the generic context switching code can be used from any
  domain, serializing execution as required.

These changes have to be paired with arch-specific code further
enabling context switching from the head domain.