When not sending data to a subprocess, close its stdin
When not sending data to a subprocess, close its stdin

We were letting stdin untouched, which meant child processes had
access to our terminal input. That meant `!fmt` was trying to read
from our terminal input and catching keystrokes.

Fixes #1281

Somasis/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,mawww/kakoune,danr/kakoune,alexherbo2/kakoune,jjthrash/kakoune,lenormf/kakoune,mawww/kakoune,Somasis/kakoune,lenormf/kakoune,casimir/kakoune,Somasis/kakoune,lenormf/kakoune,alexherbo2/kakoune,lenormf/kakoune,Somasis/kakoune,jjthrash/kakoune,jjthrash/kakoune,occivink/kakoune,danr/kakoune,danr/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,jjthrash/kakoune,casimir/kakoune,mawww/kakoune,casimir/kakoune,mawww/kakoune,occivink/kakoune,occivink/kakoune,danr/kakoune,occivink/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,casimir/kakoune