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/src/die.h

@@ -0,0 +1,31 @@
+/* Report an error and exit.
+   Copyright 2016-2017 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, write to the Free Software
+   Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA
+   02110-1301, USA.  */
+
+#ifndef DIE_H
+# define DIE_H
+
+# include <error.h>
+# include <stdbool.h>
+# include <verify.h>
+
+/* Like 'error (STATUS, ...)', except STATUS must be a nonzero constant.
+   This may pacify the compiler or help it generate better code.  */
+# define die(status, ...) \
+  verify_expr (status, (error (status, __VA_ARGS__), assume (false)))
+
+#endif /* DIE_H */