[gtp] Interrupting commands + GMP to GTP adapter

thanks to all people who sent me private email to discuss this topic.

I finally introduced the following convention to allow interrupting commands:

If the controlling program sends a special comment line (# interrupt), while a command is running, the engine should interrupt the running command (which success or failure whatever makes sense).

This allows engines to implement interrupting, while engines who don't support this convention will read the comment line after the current command is finished (and simply ignore it).

I adopted this convention for GoGui, if the interrupt button on the toolbar is pressed. GoGui also includes a GMP to GTP adapter program now, but the implementation is still in an early stage.

- Markus