streams: introduce setbuf hooks.

* include/mailutils/sys/stream.h (_mu_stream) <setbuf_hook>: New member.
* libmailutils/stream/stream.c (mu_stream_set_buffer): Use setbuf_hook
to decide whether the operation is allowed.

@@ -69,6 +69,8 @@ struct _mu_stream
   int  event_mask;
   void *event_cb_data;
 
+  int (*setbuf_hook) (mu_stream_t, enum mu_buffer_type, size_t);
+  
   const char *(*error_string) (struct _mu_stream *, int);
   
 };

@@ -554,6 +554,13 @@ mu_stream_set_buffer (mu_stream_t stream, enum mu_buffer_type type,
   if (size == 0)
     size = mu_stream_default_buffer_size;
 
+  if (stream->setbuf_hook)
+    {
+      int rc = stream->setbuf_hook (stream, type, size);
+      if (rc)
+	return rc;
+    }
+  
   if (stream->buffer)
     {
       mu_stream_flush (stream);