Fix some bugs with column width

[peterfab9845]

• In the vertical columns mode, newlines were added based on a repeat of the previous item's width, even though that is not necessarily true.
• If the last character of a filename that fits in the given space is doublewide, it was not cut off correctly.

I've also noticed the following issues, but I'm not sure exactly how to fix them:

• Using columns (-C) without vertical columns mode prefers cutting off filenames to stay in two columns rather than going to one column. (Maybe intended - if so, there is another bug adding newlines when filenames need to be cut off.)
• In vertical columns mode, filenames are not cut off to fit when only one column is possible, instead causing native wrapping by the terminal.