Apply the following:
1. Switch to `matchadd` as `match` can only be used for **one**
highlight
2. Use an autocommand group for better behaviour when reloading the
vimrc.
3. Use just one pattern as it seems to work always. (Apparently `\%#` is
not matched in normal mode)
As relative line numbers are only really needed in normal mode and in
the current buffer, disable them in insert mode or when the
buffer/window is not focused.
The old `textwidth` value is meant to be subtracted from colorcolumn, to
then add the new one.
Fixes: 41656d28d6 ("vim:autocmd: Reset textwidth value in colorcolumn")
