Support bidi reordering of unibyte strings. Fix crash displaying "All" in mode line...

Support bidi reordering of unibyte strings.  Fix crash displaying "All" in mode line of an empty buffer.

 src/dispextern.h (struct bidi_string_data): New member `unibyte'.
 src/xdisp.c (handle_single_display_spec, next_overlay_string)
 (get_overlay_strings_1, reseat_1, reseat_to_string)
 (push_display_prop): Set up the `unibyte' member of bidi_it.string
 correctly.  Don't assume unibyte strings are not bidi-reordered.
 (compute_display_string_pos)
 (compute_display_string_end): Fix handling the case of C string.
 src/bidi.c (bidi_count_bytes, bidi_char_at_pos): Accept an
 additional argument UNIBYTE, and support unibyte strings.  All
 callers changed.
 (bidi_fetch_char): Support unibyte strings.