Make process exit status more flexible to allow for handling 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.

Helpers for common use-cases are in `std::rt::io::process`.

Should resolve https://github.com/mozilla/rust/issues/10062.