Ignore SIGINT sent to an interactive repl server

Unfortunately, even though `*pump-in*` is false, the server subprocess still gets the SIGINT when you hit `<ctrl-c>`. REPLy used to take care of that by registering a handler itself, but it's a ClassLoader leak, as
discussed in trptcolin/reply#127, so I removed it. But now subprocess servers crash instead of gracefully being interrupted.

Here we're careful only to register the process in the specific case where we're responsible for both the input and the server.

Pro: REPLy clients avoid classloader leaks.
Con: All REPLy clients have to implement this same sort of handling, but only (?) if they're running in a subprocess.