It's possible that a child process won't close it's standard streams, even after
it exits. This can be evidenced with the "xclip" program:
:call system('xclip -i -selection clipboard', 'DATA')
Before this commit, the above command wouldn't return, even though the xclip
program had exited. That is because `xclip` wasn't closing it's stdout/stderr
streams, which would block pending_refs from ever reaching 0.
Now the job.c module was refactored to ensure all streams are closed when the
uv_process_t handle is closed.