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/vasprintf.m4

@@ -0,0 +1,46 @@
+# vasprintf.m4 serial 6
+dnl Copyright (C) 2002-2003, 2006-2007, 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_FUNC_VASPRINTF],
+[
+  AC_CHECK_FUNCS([vasprintf])
+  if test $ac_cv_func_vasprintf = no; then
+    gl_REPLACE_VASPRINTF
+  fi
+])
+
+AC_DEFUN([gl_REPLACE_VASPRINTF],
+[
+  AC_LIBOBJ([vasprintf])
+  AC_LIBOBJ([asprintf])
+  AC_REQUIRE([gl_STDIO_H_DEFAULTS])
+  if test $ac_cv_func_vasprintf = yes; then
+    REPLACE_VASPRINTF=1
+  else
+    HAVE_VASPRINTF=0
+  fi
+  gl_PREREQ_VASPRINTF_H
+  gl_PREREQ_VASPRINTF
+  gl_PREREQ_ASPRINTF
+])
+
+# Prerequisites of the vasprintf portion of lib/stdio.h.
+AC_DEFUN([gl_PREREQ_VASPRINTF_H],
+[
+  dnl Persuade glibc <stdio.h> to declare asprintf() and vasprintf().
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+])
+
+# Prerequisites of lib/vasprintf.c.
+AC_DEFUN([gl_PREREQ_VASPRINTF],
+[
+])
+
+# Prerequisites of lib/asprintf.c.
+AC_DEFUN([gl_PREREQ_ASPRINTF],
+[
+])