Commit @45525853d352 removed usage of the `job_write_cb` for closing stdin due
to a memory error, but that doesn't work anymore because `job_close_in` closes
stdin immediately, possibly trimming input data before it is fully written.
Since most memory issues with jobs have been fixed, re-add the `job_write_cb`
call to ensure stdin is only closed when it should. Also add tests for scenarios
where using the callback makes a difference.
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.
