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/quote.m4

@@ -0,0 +1,13 @@
+# quote.m4 serial 6
+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_QUOTE],
+[
+  dnl Prerequisites of lib/quote.c.
+  dnl (none)
+  :
+])