Use keyword with R6RS mode

keyword.scm

#!r6rs
(import (rnrs))
(begin
  #!compatible
  (call-with-input-file "test.scm"
    (lambda (p) (print (get-bytevector-all p)))
    :transcoder #f)
  #!r6rs
  (print (keyword? :key)) ;; error unbound variable
)