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/pathmax.m4

@@ -0,0 +1,42 @@
+# pathmax.m4 serial 10
+dnl Copyright (C) 2002-2003, 2005-2006, 2009-2017 Free Software Foundation,
+dnl 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_PATHMAX],
+[
+  dnl Prerequisites of lib/pathmax.h.
+  AC_CHECK_HEADERS_ONCE([sys/param.h])
+])
+
+# Expands to a piece of C program that defines PATH_MAX in the same way as
+# "pathmax.h" will do.
+AC_DEFUN([gl_PATHMAX_SNIPPET], [[
+/* Arrange to define PATH_MAX, like "pathmax.h" does. */
+#if HAVE_UNISTD_H
+# include <unistd.h>
+#endif
+#include <limits.h>
+#if defined HAVE_SYS_PARAM_H && !defined PATH_MAX && !defined MAXPATHLEN
+# include <sys/param.h>
+#endif
+#if !defined PATH_MAX && defined MAXPATHLEN
+# define PATH_MAX MAXPATHLEN
+#endif
+#ifdef __hpux
+# undef PATH_MAX
+# define PATH_MAX 1024
+#endif
+#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
+# undef PATH_MAX
+# define PATH_MAX 260
+#endif
+]])
+
+# Prerequisites of gl_PATHMAX_SNIPPET.
+AC_DEFUN([gl_PATHMAX_SNIPPET_PREREQ],
+[
+  AC_CHECK_HEADERS_ONCE([unistd.h sys/param.h])
+])