SIGINT on Ctrl-C in REPL if under ptrace

This can just as well live in my .juliarc, but I figured I would see if anyone else is interested. This raises `SIGINT` upon `Ctrl-C` in the REPL if Julia is running under ptrace, which is nice for setting breaks in the debugger (^Z works, but it is annoying because it takes 3 or 4 continues to resume due to threads).