default values for updating border and cursor colors

[jackkamm]

Many Xresources (for example, https://github.com/chriskempson/base16-xresources) do not set the border and cursor colors, in which case they are taken from the background and foreground respectively. However urxvt-config-reload only updates these colors when explicitly set in Xresources. This PR changes the behavior so these colors get updated, even when not explicitly set.