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/lib/prepargs.h

@@ -0,0 +1,3 @@
+/* Parse arguments from a string and prepend them to an argv.  */
+
+void prepend_default_options (char const *, int *, char ***);