This is needed for Neovim, because an external UI could be attached to
the same neovim server, so it does not make sense to define highlighting
groups with either only the cterm or the guifg attribute set.
So refactor the code slightly got get rid of this variable (and since
this variable is not needed anymore, we can also get rid of the guienter
and OptionSet autocommand).
fixes: #2261
This is required when used with 'Normal' with a transparent background.
Falling back to 1 here results in "red" for "ctermbg"!
I think it's important to keep the 'NONE' color property here, instead
of hardcoding the fallbacks (even if white would be used instead of
red).
