events: Rework event subsystem (add_event, send_event) to be more versatile.

add_event_ex is added that takes an extra user_data pointer. This pointer is
passed to the callback (add_event and add_event_ex have slightly different
callbacks types). All callbacks also get the event id passed. Events added
with add_event_ex must be removed with remove_event_ex because the user_data
pointer must match in addition to the callback pointer.

On the other add_event is simplified to omit the oneshort parameter which
was almost always false (still there with add_event_ex).

As a side effect the ata_idle_notify callbacks are changed as well, they
do not take a data parameter anymore which was always NULL anyway.

This commit also adds some documentation to events.h

Change-Id: I13e29a0f88ef908f175b376d83550f9e0231f772
Fix reds.

Change-Id: I4f065a5b9f96df4d70ee8bb2836effc9b97b01fa
Fix stupid typo in 470989b that triggered panics easily.

Change-Id: Ic998a0ea93141a5399e28dc286eea4e3d40663d6
Merge branch 'master' into lab-general