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/allocator.c

@@ -0,0 +1,5 @@
+#define _GL_USE_STDLIB_ALLOC 1
+#include <config.h>
+#include "allocator.h"
+#include <stdlib.h>
+struct allocator const stdlib_allocator = { malloc, realloc, free, NULL };