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/asnprintf.c

@@ -0,0 +1,34 @@
+/* Formatted output to strings.
+   Copyright (C) 1999, 2002, 2006, 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>
+
+/* Specification.  */
+#include "vasnprintf.h"
+
+#include <stdarg.h>
+
+char *
+asnprintf (char *resultbuf, size_t *lengthp, const char *format, ...)
+{
+  va_list args;
+  char *result;
+
+  va_start (args, format);
+  result = vasnprintf (resultbuf, lengthp, format, args);
+  va_end (args);
+  return result;
+}