at 1 GHZ the intermediate calculation for cycles overflows 32 bits
this makes timer fail even with sensible values
solution divide both sides by 100
Change-Id: I18a4054c2d06fb72531d5496bba562f71b03984f
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
