I sometimes work in files where I cannot remove trailing whitespaces (at
least not permanently). In these cases the background highlighting of
them is quite strong and can be a bit annoying. To make this more
pleasing, use the foreground to highlight trailing characters, as long
as tabs and trailing spaces are displayed as non-space characters.
For this I also set `trail` in `&listchars`, and while at it, merged the
two lines setting `&listchars` and cleaned up some comments.
