Unix: require confirmation before overwriting containers

[felix314159]

Overview

Before this PR --text --create replaced an existing file container without confirmation and without requiring --force. That should not be possible.

Reproduction

v=./src/Main/veracrypt
p=/tmp/existing.hc

$v --text --create "$p" --size=4M \
  --password='Original-Password-For-Test' --pim=0 --keyfiles='' \
  --hash=sha512 --encryption=aes --filesystem=none \
  --volume-type=normal --quick --random-source=/dev/urandom \
  --non-interactive

sha256sum "$p"

# Previously succeeded and changed the hash without --force.
$v --text --create "$p" --size=4M \
  --password='Replacement-Password-For-Test' --pim=0 --keyfiles='' \
  --hash=sha512 --encryption=aes --filesystem=none \
  --volume-type=normal --quick --random-source=/dev/urandom \
  --non-interactive

sha256sum "$p"

This change makes noninteractive creation reject an existing normal container unless --force is supplied. Interactive text mode now displays the existing overwrite warning and requests confirmation, matching the GUI.

I validated the fix with a text-mode Linux build, algorithm self-tests, and overwrite tests both with and without --force.