racket/system: add some missing synchronization

These changes are intended to address "input port is closed" errors
that have been showing up with Racket CS, possibly because its
scheduler exposed missing synchronization.

racket/collects/racket/private/streams.rkt

@@ -25,17 +25,19 @@
 
 (define (streamify-in cin in ready-for-break)
   (if (and cin (not (file-stream-port? cin)))
-    (thread (lambda ()
-              (dynamic-wind
-                void
-                (lambda ()
-                  (with-handlers ([exn:break? void])
-                    (ready-for-break #t)
-                    (copy-port cin in)
-                    (ready-for-break #f)))
-                (lambda () (close-output-port in)))
-              (ready-for-break #t)))
-    in))
+      (parameterize-break #f
+        (thread (lambda ()
+                  (dynamic-wind
+                   void
+                   (lambda ()
+                     (with-handlers ([exn:break? void])
+                       (parameterize-break #t
+                         (ready-for-break #t)
+                         (copy-port cin in)
+                         (ready-for-break #f))))
+                   (lambda () (close-output-port in)))
+                  (ready-for-break #t))))
+      in))
 
 (define (streamify-out cout out)
   (if (and cout 

racket/collects/racket/system.rkt

@@ -195,7 +195,8 @@
         (when it
           ;; stop piping output to subprocess
           (semaphore-wait it-ready)
-          (break-thread it))
+          (break-thread it)
+          (thread-wait it))
         ;; wait for other pipes to run dry:
         (when (thread? ot) (thread-wait ot))
         (when (thread? et) (thread-wait et))