console.lua: repurpose case_sensitive to only affect exact searches

case_sensitive originally only affected Tab completion. It defaulted to
yes outside of Windows just to emulate the bad defaults of bash and zsh
(not fish, which has better defaults and defaults to case-insensitive
completion). This was already silly, but then it affected the fuzzy
autocompletion and case-sensitive fuzzy completion is even more silly.
And since 0b3cc3a167, it accidentally stopped affecting even that,
because the completion via script message never checked for the option.
Nobody even noticed this, meaning that nobody was relying on fuzzy
autocompletion being case-sensitive.

Just make case_sensitive only affect the new exact search, and make it
default to no on all platforms. Though IMO the search could just always
be case-insensitive.