Override readline's getc function to use Racket's read-byte.
Override readline's getc function to use Racket's read-byte.

Eliminates the busy-polling behavior observed in Mac OS X when
xrepl is required at the regular repl.

Closes PR 13350.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket