Read whole line in yes_or_no

Read whole line in yes_or_no

Do not stop after 79 characters. Read the complete line to avoid
arbitrary limitations.

Proof of Concept:

```
cat > passwd-poc << EOF
root:x:0:0:root:/root:/bin/bash
root:x:0:0:root:/root:/bin/bash
root:x:0:0:root:/root:/bin/bash
EOF
python -c "print(80*'y')" | pwck passwd-poc
```

Two lines should still be within the file because we agreed only once
to remove a duplicated line.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>
Reviewed-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Serge Hallyn <serge@hallyn.com>