Add exit

[ezrosent]

This PR adds support for the exit command to frawk, which should close out #71

Most of the complexity here is around providing useful semantics for exit when it is invoked from a parallel script. The overall strategy here is that if exit is called from a worker thread it will cut off input for all other threads, leading them to hopefully exit early. At that point, the main thread can do an orderly shutdown with the proper exit code.

[jicksaw]

@ezrosent https://github.com/ezrosent/frawk/blob/master/info/overview.md#what-is-missing still says that exit isn't supported

[ezrosent]

Thank you for catching that! Updated the text.