Close stdin
Close stdin

Ensures exec doesn't hang waiting for stdin file descriptor to close.

Solves following use case:

`mongod.sh` : 

```sh
#!/usr/bin/env bash

docker run -i -p 27017:27017 --rm --entrypoint=mongod mongo $@
```

`$ ln -s $PWD/mongod.sh /bin/mongod`

If you don't close stdin, then `learnyoumongo verify` will hang indefinitely.
jqk6/learnyoumongo,venturads/learnyoumongo,metakata/learnyoumongo,cybot1711/learnyoumongo,KarteekRakshit/learnyoumongo,evanlucas/learnyoumongo,WilBeCoding/learnyoumongo