Pass up termination signal instead of zero exit status when a child is terminated by signal

The UvProcess exit callback is called with a zero exit status and non-zero termination signal when a child is terminated by a signal.

If a parent checks only the exit status (for example, only checks the return value from `wait()`), it may believe the process completed successfully when it actually failed.

So far, this has been an issue on only OSX in my testing; Linux does not seem to hit the exit callback at all.

Should be enough to resolve https://github.com/mozilla/rust/issues/10062, but is this potentially too naive a fix?