Factor target_timer_callbacks_check_time into pieces:

- Add target_timer_callback_periodic_restart and target_call_timer_callback.
- Clean up and simplify logic that determines whether to call each callback. 
- Move variable declarations to location of first use.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2195 b42882b7-edfa-0310-969c-e2dbd0fdcd60