If the shell spawns a subprocess which doesn't close the output file
descriptors, the exec.Cmd will block on Wait() (see
golang.org/issue/18874). Use an os.Pipe to provide the command with a
real file descriptor so the exec package doesn't need to do the copy
manually. This in turn may block our own reading goroutine, but we can
select on that and leave it for cleanup later.
James Bardin