Scheduling policies may prevent threads from running based on some
extra state, by not returning them from the sched_pick() handler,
without reflecting such state with any block bit. Those threads remain
in the ready state from the scheduler core point of view, although
they won't be elected for running.

This behavior is typical of policies enforcing a runtime budget for
instance.

However, we also have to honor the request to move a thread out of
primary mode (i.e. shadow kicking). Therefore we need a way to tell
the policy module to release such thread temporarily, until it
eventually relaxes (e.g. sigwake -> kick -> relax).

To this end, the sched_kick() handler can be defined by policy modules
for dealing with this specific case. It is called for kicked threads
bearing the XNREADY bit, once all block bits have been lifted.