Update set_option calls to use the recommended APIs

[neilvyas]

I was having an issue where fillchars would be overriden when I exited zenmode. It turns out that this was actually an issue in another plugin---I had ZenMode change colorschemes with a hook, and actually that set an option in an unsafe way---but I assumed that ZenMode was the source of the problem and wound up writing this code before tracing the source it to the other plugin, so I figured that I might as well upstream it.