Arrange for on_exit_nicely to be thread-safe.

Extracted from Joachim Wieland's parallel pg_dump patch, with some
additional comments by me.

src/bin/pg_dump/dumputils.c

@@ -1319,16 +1319,18 @@ on_exit_nicely(on_exit_nicely_callback function, void *arg)
 	on_exit_nicely_index++;
 }
 
-/* Run accumulated on_exit_nicely callbacks and then exit quietly. */
+/*
+ * Run accumulated on_exit_nicely callbacks in reverse order and then exit
+ * quietly.  This needs to be thread-safe.
+ */
 void
 exit_nicely(int code)
 {
-	while (--on_exit_nicely_index >= 0)
-		(*on_exit_nicely_list[on_exit_nicely_index].function)(code,
-			on_exit_nicely_list[on_exit_nicely_index].arg);
-#ifdef WIN32
-	if (parallel_init_done && GetCurrentThreadId() != mainThreadId)
-		ExitThread(code);
-#endif
+	int		i;
+
+	for (i = on_exit_nicely_index - 1; i >= 0; i--)
+		(*on_exit_nicely_list[i].function)(code,
+			on_exit_nicely_list[i].arg);
+
 	exit(code);
 }