Disable GCL readline symbol completion by setting SI::*READLINE-PREFIX* to a nonexistent package.
Fixes SF bug #3972: "gcl only: Autocompletion inverts command case"
This is a suboptimal bug fix. As it says in the comments,
;; This is kind of terrible. I don't see a flag to only disable completion,
;; or a way to set the symbol list to Maxima symbols and disable case inversion,
;; so set the completion prefix to a nonexistent package.
;; If ever package BLURFLE is actually defined, and contains external symbols,
;; those symbols will be completed. I can live with that.