Multi-line paste warning doesn't detect \r, only \n

[DHowett]

Certain shells will execute commands on \r as well as \n, so we should try to catch \r as well.

This should probably be tackled at the same time as paste filtering. I originally thought that we should detect newlines after filtering, or perhaps during filtering, but would result in us doing a "lot" of work in the case where the user declines the paste.

Filtering: #5821.

[DHowett]

@PankajBhojwani has expressed some interest in the various multiline paste issues. 😄