D7: editor multi-cursor support

Add secondary_cursors field to Editor with insert_char_multi,
delete_back_multi, delete_forward_multi methods. Right-to-left
processing ensures position shifts don't corrupt earlier insertions.

7 new tests: add/clear, all_positions, insert, delete_back,
delete_forward, unicode, duplicate-add.

local/recipes/tools/diffutils/source/lib/strndup.c

@@ -0,0 +1,36 @@
+/* A replacement function, for systems that lack strndup.
+
+   Copyright (C) 1996-1998, 2001-2003, 2005-2007, 2009-2017 Free Software
+   Foundation, Inc.
+
+   This program 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.
+
+   This program 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 this program; if not, see <http://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include <string.h>
+
+#include <stdlib.h>
+
+char *
+strndup (char const *s, size_t n)
+{
+  size_t len = strnlen (s, n);
+  char *new = malloc (len + 1);
+
+  if (new == NULL)
+    return NULL;
+
+  new[len] = '\0';
+  return memcpy (new, s, len);
+}