Add display option to disable username highlighting

[milosivanovic]

In htop 3.3.0, username highlighting was added as per https://github.com/htop-dev/htop/blob/3.3.0/htop.1.in#L457-L460.

This use of colour/highlighting should be optional with a dedicated menu item in "F2 -> Display options". Making the username highlighting optional would be consistent with other new uses of colour in htop (see https://github.com/htop-dev/htop/issues/383.)