Print ClearScreenAfterCursor on shutdown

There are scenarios in which sbt can leave behind stray progress lines
after it exits. This attempts to delete them.

main/src/main/scala/sbt/Main.scala

@@ -102,7 +102,10 @@ private[sbt] object xMain {
         }
       }
     } finally {
+      // Clear any stray progress lines
       ShutdownHooks.close()
+      System.out.print(ConsoleAppender.ClearScreenAfterCursor)
+      System.out.flush()
     }
   }