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/m4/freopen.m4

@@ -0,0 +1,40 @@
+# freopen.m4 serial 6
+dnl Copyright (C) 2007-2017 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_FUNC_FREOPEN],
+[
+  AC_REQUIRE([gl_STDIO_H_DEFAULTS])
+  AC_REQUIRE([AC_CANONICAL_HOST])
+  case "$host_os" in
+    mingw* | pw* | os2*)
+      REPLACE_FREOPEN=1
+      ;;
+    *)
+      AC_CACHE_CHECK([whether freopen works on closed fds],
+        [gl_cv_func_freopen_works_on_closed],
+        [AC_RUN_IFELSE(
+           [AC_LANG_PROGRAM(
+              [[#include <stdio.h>
+                #include <unistd.h>
+              ]],
+              [[close (0);
+                return !(freopen ("/dev/null", "r", stdin)
+                         && getchar () == EOF
+                         && !ferror (stdin) && feof (stdin));]])],
+           [gl_cv_func_freopen_works_on_closed=yes],
+           [gl_cv_func_freopen_works_on_closed=no],
+           [case $host_os in
+              *gnu*) gl_cv_func_freopen_works_on_closed="guessing no" ;;
+              *)     gl_cv_func_freopen_works_on_closed="guessing yes";;
+            esac])])
+      case $gl_cv_func_freopen_works_on_closed in
+        *no) REPLACE_FREOPEN=1;;
+      esac
+  esac
+])
+
+# Prerequisites of lib/freopen.c.
+AC_DEFUN([gl_PREREQ_FREOPEN], [:])