Fix timer Agptek Rocker (other hosted players)

on timer_unregister callbacks are not removed

It seems (at least on the Rocker) timers continue to fire (for a bit??)

Now we store the registered callback in the sigev structure and check
that the callback matches the one registered when the timer is created.

This should stop the possible case of a new timer getting spurious callbacks
We also now NULL the callbacks on un-register which should stop the segfaults

Added some notes to timer.c and timer.h

Change-Id: Ia155c3a4e4af89f474d55ed845560ccc1fab85aa
3 files changed