main : Fix Ctrl+D/newline handling (#12951)
This restores the behavior from #491. This does not affect Ctrl+D's ability to
terminate --multiline-input lines (#1040).
This also actually implements #587: "If the user wants the text to end in a
newline, this should be accomplished by explicitly adding a newline by using
\ followed by return, then returning control by pressing return again."
Fixes #12949
