bug#12216: peek-char incorrectly *CONSUMES* eof

[David A. Wheeler]

I reported:
> > Guile's peek-char has a bug; it incorrectly *consumes* eof instead of
> > just reporting it.

Andy Wingo replied:
> I have the feeling that for interactive use, if you expect to read a EOF
> from a port and then continue, you have to not use peek-char.  You need
> to handle your own lookahead buffer.  I know it's not a great answer,
> but I can't think of anything else that makes sense.

We don't want to read an EOF and then continue.
We peek-char to not *CONSUME* an interactive EOF.
Once peek-char returns EOF, every following peek-char
should *also* return EOF until the EOF is read.
That isn't what currently happens.

E.G.:
(define (demo) (write (peek-char)) (write (peek-char)))

(demo)

; Now type control-D ENTER.
; It SHOULD produce #<eof> twice, after peeking EOF,
; but instead it produces #<eof> #\newline
#<eof>#<eof>guile> (demo)

--- David A. Wheeler