sched: Directly pass context pointer to pre/post_select().

sched: Directly pass context pointer to pre/post_select().

The patch is large, but it's fairly straight forward: Instead of
a task pointer all ->pre_select() and ->post_select() methods now
receive the context pointer that was passed to the scheduler when
the task was registered. This allows to kill the public task_context().

Two pre_select/post_select functions are not directly called by the
scheduler: session_post_select(), generic_recv_pre_select(). These
are changed to receive a proper struct rather than a void pointer.

Note that generic_filter_pre_select() is not changed in this
manner because some filters do not provide a pre_select wrapper but set
task->pre_select to generic_filter_pre_select().