Clean up jtag command queue handling:

Clean up jtag command queue handling:
- Rename last_command_pointer as next_command_pointer, because this variable
  stores the address where jtag_queue_command() will store a command pointer.
- Make that variable static, since it is only used internally in jtag.c.
- Remove superfluous accessor for that now-static variable.
- Deobfuscate use of variables in jtag_command_queue.
- Add jtag_command_queue_reset helper function.
- Use it in interface_jtag_execute_queue.

git-svn-id: svn://svn.berlios.de/openocd/trunk@1992 b42882b7-edfa-0310-969c-e2dbd0fdcd60