Implemented a ProcessExit enum and helper methods to std::rt::io::process for getting process termination status, or the signal that terminated a process.

A test has been added to rtio-processes.rs to ensure signal termination is picked up correctly.
auto merge of #10109 : pcmattman/rust/pass-nonzero-exit-status-on-termination-by-signal, r=alexcrichton

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.