This add the missing cursor move to the top left in case the upper
window is selected as stated in z-spec 8.7.2.1.1. This will make screen
refreshes work again for Seastalker.

Since the blockbuffer doesn't shrink, the buffer contents must not be
realigned and buffer sizes must not be reduced in case the screen gets
smaller.

Blockbuffer will always work, even before split_window has been invoked.
Made fizmo_new_screen_size adapt blockbuffer screen size. The blockbuffer is
now automatically created upon startup.

- 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.

- Merged non-NULL z_mem verification in fizmo_new_screen_size().
- Merged non-NULL z_mem check in write_interpreter_info_into_header().
- Merged max-undo-steps evaluation and z_mem-NULL-reset in fizmo_start().
- Fixed erroneous xdg_config_home-NULL-check in fizmo_start().