cs: auto-flush stdout/stderr only when terminals

Match BC's long-standing behavior of only flushing the original output
and error ports on input from the original input port when the output
and/or error port is a terminal port.

pkgs/racket-doc/scribblings/reference/port-buffers.scrbl

@@ -43,10 +43,12 @@ one byte is read for @racket[read-bytes-avail!*],
 buffered bytes, but no further bytes are read.
 
 In addition, the initial current output and error ports are
-automatically flushed when @racket[read], @racket[read-line],
+automatically flushed when they are terminal ports (see
+@racket[terminal-port?]) and when @racket[read], @racket[read-line],
 @racket[read-bytes], @racket[read-string], etc., are performed on the
-initial standard input port; more precisely, flushing is performed by
-the default port read handler (see @racket[port-read-handler]).
+initial standard input port. (More precisely, instead of
+@racket[read], flushing is performed by the default port read handler;
+see @racket[port-read-handler].)
 
 @defproc[(flush-output [out output-port? (current-output-port)]) void?]{
 

racket/src/cs/schemified/io.scm

@@ -11890,8 +11890,12 @@
   (lambda (in_0)
     (if (eq? in_0 (unsafe-place-local-ref cell.1$9))
       (begin
-        (1/flush-output (unsafe-place-local-ref cell.2$2))
-        (1/flush-output (unsafe-place-local-ref cell.3)))
+        (if (1/terminal-port? (unsafe-place-local-ref cell.2$2))
+          (1/flush-output (unsafe-place-local-ref cell.2$2))
+          (void))
+        (if (1/terminal-port? (unsafe-place-local-ref cell.3))
+          (1/flush-output (unsafe-place-local-ref cell.3))
+          (void)))
       (void))))
 (define do-read-bytes!
   (lambda (who_0 in_0 bstr_0 start_0 end_0)

racket/src/io/port/flush-output.rkt

@@ -6,7 +6,8 @@
          "port.rkt"
          "output-port.rkt"
          "pipe.rkt"
-         "check.rkt")
+         "check.rkt"
+         "fd-port.rkt")
 
 (provide flush-output
          maybe-flush-stdout)
@@ -37,5 +38,7 @@
 
 (define (maybe-flush-stdout in)
   (when (eq? in orig-input-port)
-    (flush-output orig-output-port)
-    (flush-output orig-error-port)))
+    (when (terminal-port? orig-output-port)
+      (flush-output orig-output-port))
+    (when (terminal-port? orig-error-port)
+      (flush-output orig-error-port))))