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/gnulib-tests/test-strerror.c

@@ -0,0 +1,75 @@
+/* Test of strerror() function.
+   Copyright (C) 2007-2017 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/>.  */
+
+/* Written by Eric Blake <ebb9@byu.net>, 2007.  */
+
+#include <config.h>
+
+#include <string.h>
+
+#include "signature.h"
+SIGNATURE_CHECK (strerror, char *, (int));
+
+#include <errno.h>
+
+#include "macros.h"
+
+int
+main (void)
+{
+  char *str;
+
+  errno = 0;
+  str = strerror (EACCES);
+  ASSERT (str);
+  ASSERT (*str);
+  ASSERT (errno == 0);
+
+  errno = 0;
+  str = strerror (ETIMEDOUT);
+  ASSERT (str);
+  ASSERT (*str);
+  ASSERT (errno == 0);
+
+  errno = 0;
+  str = strerror (EOVERFLOW);
+  ASSERT (str);
+  ASSERT (*str);
+  ASSERT (errno == 0);
+
+  /* POSIX requires strerror (0) to succeed.  Reject use of "Unknown
+     error", but allow "Success", "No error", or even Solaris' "Error
+     0" which are distinct patterns from true out-of-range strings.
+     http://austingroupbugs.net/view.php?id=382  */
+  errno = 0;
+  str = strerror (0);
+  ASSERT (str);
+  ASSERT (*str);
+  ASSERT (errno == 0);
+  ASSERT (strstr (str, "nknown") == NULL);
+  ASSERT (strstr (str, "ndefined") == NULL);
+
+  /* POSIX requires strerror to produce a non-NULL result for all
+     inputs; as an extension, we also guarantee a non-empty result.
+     Reporting EINVAL is optional.  */
+  errno = 0;
+  str = strerror (-3);
+  ASSERT (str);
+  ASSERT (*str);
+  ASSERT (errno == 0 || errno == EINVAL);
+
+  return 0;
+}