We don't have full-fledged in-kernel APIs going in and out selectively
anymore, we now have a stable set of API services (i.e. core, POSIX
and RTDM), with optional limited extensions through Xenomai
"personalities".

For this reason, the former "pod" abstraction, as a mean to group
multiple API siblings in a common generic container is not much
relevant anymore.

The ongoing design simplification now allows to drop the pod-related
services, dispatching them to the other existing abstractions.

The renaming involved are fairly straightforward for the most part,
i.e.:

* Former thread-directed ops:
  xnpod_<action>_thread() => xnthread_<action>()

* Former scheduler-related ops:
  xnpod_<action>() => xnsched_<action>()

Hint: xnpod_schedule() became xnsched_run() (although xnsched_ule()
would have been delighting).