cmd/a+c: add a+c tool

cmd/a+c: add a+c tool This is the program we've been using as 'addca' for quite a while. It has lived in my home directory at work. I needed to update it today and discovered that there is no reason not to publish it (the equivalent used by Chrome developers is already public). The program can still only be used by Google employees, but putting it in the repo makes it easier to collaborate on its development.