Add jtag_event_handler_t:

- Define the function signature used by the JTAG event callback mechanism.
- Provide Doxygen block for new type, including TODO for its return value.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2168 b42882b7-edfa-0310-969c-e2dbd0fdcd60