Examine output of external command

[sakhnik]

Previously it was possible to examine output of an external command (make, for instance) in the terminal. With recent changes to neovim, this wouldn't work any more:

1. Start nvim
2. Execute :!for i in $(seq 1 1000); do echo $i; done
3. Type ^Z

Expectation: The terminal shows listed numbers.
Reality: No output from external commands launched from neovim.

[justinmk]

see #1716