Implementation of process::io::poll can lead to broken pipes.

When a future returned from process::io::poll is discarded we don't properly remove the file descriptors from the kernel/libev which can cause unintended side effects depending on how the file descriptor is reused.

See this TODO: https://github.com/apache/mesos/commit/53a3f4158194aff462f5b3adb3d9d220156a5e12#diff-a8849aee70f7a97204bf8ef5ff2117abR3604