Merge zarf's maxundostep changes, adapted cleanup.

- Merged/adapted set_max_undo_steps().
- Merged opcode_save_undo() changes.
- Fixed segfault in case undo_index == max_undo_steps.
- Adapted get_allocated_undo_memory_size, added size of undo_frames to
  memory amount.
- Adapted free_undo_memory.