Don't flood stdout with unanswered question

I happen to run dput from a script (via rebuildd), and I guess that stdin is closed by rebuildd. In that case, the question(s) asked by dput-ng is asked non-stop in a wild loop, as sys.stdin.realine() returns immediately.

To reproduce: first assume that you try to upload to a host that is unknown (not present in known_hosts), dput will prompt as such:

please login: To accept ssh-ed25519 hostkey b'<...>' for repo.kali.org type "yes": [yes, no]:

Now run this script:

#!/bin/sh
exec 0</dev/null
dput -U <host> <some-package>.changes

And witness the flood, which either wastes your terminal, or quickly fills up gigabytes if stdout is redirected.

This commit limits the damage.