Plugins (YCM, dispatch.vim) use gui_running to decide behavior; so do
some colorschemes. Up to now, nvim lied about gui_running for the
benefit of colorschemes and the detriment of all _other_ plugins that
check this condition. That's counterproductive: a user employs at most
_one_ colorscheme but may use many other plugins which expect the Vim
legacy semantics of gui_running.
Moreover, colorschemes usually don't _need_ to check gui_running: they
can instead set cterm{fg,bg} and gui{fg,bg} in the same :highlight call.
It is reasonable for users who want "true color" to modify their
colorscheme once (or request upstream to do so) in order to avoid
running into quirks in any other plugins.
Closes #2782