drracket: plumber for user process

The exit handler in the user process flushes via the plumber,
but maybe DrRacket could do more with the user's plumber, such as
attempting to exit instead of always forcing the old user process to
terinate when the "Run" button is clicked.

pkgs/drracket-pkgs/drracket/drracket/private/rep.rkt

@@ -1317,10 +1317,14 @@ TODO
                  (set! user-eventspace-main-thread (current-thread))
                  
                  (current-logger user-logger)
+
+                 (define user-plumber (make-plumber))
+                 (current-plumber user-plumber)
                  
                  (initialize-parameters snip-classes)
                  (let ([drracket-exit-handler
                         (λ (x)
+                          (plumber-flush-all user-plumber)
                           (parameterize-break
                            #f
                            (let ([s (make-semaphore)])