sched: Introduce task notifications.

sched: Introduce task notifications.

Currently there is no canonical way to perform inter-task
communications. This leads to ugly hacks like kill_all_decoders()
of audiod which is called when audiod switches to "off" or "standby"
mode. This function sets the error state of the receiver task to a
negative value and removes the receiver's buffer tree node, magically
knowing that this is the correct action to take in order to shut down
the receiver cleanly.

This patch provides a simple infrastructure for task communications
which makes such hacks unnecessary. Tasks may send a notification
value, which is a standard error code, to another task by calling the
new task_notify() function. The receiving task is supposed to check
for any pending notifications in its ->post_select function.

As a first application of the notification concept, the above mentioned
kill_all_decoders() function is changed to use notifications instead.