Commit missing file

+/* GNU Mailutils -- a suite of utilities for electronic mail
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   GNU Mailutils is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3, or (at your option)
+   any later version.
+
+   GNU Mailutils is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Mailutils.  If not, see <http://www.gnu.org/licenses/>. */
+
+#include <config.h>
+#include <stdlib.h>
+#include <mailutils/types.h>
+#include <mailutils/errno.h>
+#include <mailutils/list.h>
+#include <mailutils/msgset.h>
+#include <mailutils/sys/msgset.h>
+
+static int
+copy_range (void *item, void *data)
+{
+  struct mu_msgrange const *range = item;
+  mu_list_t list = data;
+  struct mu_msgrange *copy;
+
+  copy = malloc (sizeof (*copy));
+  if (!copy)
+    return ENOMEM;
+  *copy = *range;
+  return mu_list_append (list, copy);
+}
+	    
+int
+mu_msgset_copy (mu_msgset_t src, mu_msgset_t dst)
+{
+  mu_list_t list;
+  int rc;
+  
+  if (!src || !dst)
+    return EINVAL;
+
+  rc = mu_list_create (&list);
+  if (rc)
+    return rc;
+
+  rc = mu_list_foreach (src->list, copy_range, list);
+  if (rc == 0)
+    mu_list_append_list (dst->list, list);
+  mu_list_destroy (&list);
+
+  return rc;
+}