jtag_execute_queue() function

For software FIFO implementations, the queued commands can be executed during this call or earlier. A sw queue might decide to push out some of the jtag_add_xxx() operations once the queue is "big enough". This fn will return an error code if any of the prior jtag_add_xxx() calls caused a failure, e.g. check failure. Note that it does not matter if the operation was executed *before* jtag_execute_queue(), jtag_execute_queue() will still return an error code. All jtag_add_xxx() calls that have in_handler != NULL will have been executed when this fn returns, but if what has been queued only clocks data out, without reading anything back, then JTAG could be running *after* jtag_execute_queue() returns. The API does not define a way to flush a hw FIFO that runs *after* jtag_execute_queue() returns. jtag_add_xxx() commands can either be executed immediately or at some time between the jtag_add_xxx() fn call and jtag_execute_queue().