Implement stderr=STDOUT in subprocess

Author: msimacek

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/PosixSubprocessModuleBuiltins.java

@@ -175,6 +175,10 @@ private synchronized int forkExec(PList args, @SuppressWarnings("unused") PList
                 pb.redirectError(Redirect.INHERIT);
             }
 
+            if (errwrite == c2pwrite) {
+                pb.redirectErrorStream(true);
+            }
+
             try {
                 if (getContext().getEnv().getPublicTruffleFile(cwd).exists()) {
                     pb.directory(new File(cwd));