- Like Vim, use set_option_value() followed by reset_option_was_set().
- Do not use set_string_default(), so the default is predictable.
This affects `:set bg&`.
- Wait until end-of-startup (VimEnter) to handle the response. The
response is racey anyways, so timing is irrelevant. This allows
OptionSet to be triggered, unlike during startup.