Make struct sched private.

Make struct sched private.

This moves the declaration of the structure from sched.h to sched.c because
the information stored in this structure has almost no users outside of the
scheduler core.

As a result, the size of the structure is no longer known outside of sched.c.
Applications now must call the new sched_new() to obtain a pointer to an
initialized scheduler structure, passing an alternative poll function if
needed. We don't let applications set the default timeout anymore because
the value is kind of arbitrary anyway and most applications simply specified
one second. So hardcode this as the default.