Retrospective commit from 2009-09-26.

Retrospective commit from 2009-09-26.

Continued working on initialization.
Started working on paragraph direction initialization.

 bidi.c (bidi_paragraph_init): Don't set bidi_it->ch_len.  Abort
 if called not at beginning of a new paragraph.
 (bidi_get_next_char_visually): Prepare and use a sentinel iterator
 state when first_elt flag is set.
 dispextern.h (struct bidi_it): New struct member first_elt.
 bidi.c (bidi_init_it): Initialize bidi_it->first_elt.
 (bidi_copy_it): Don't copy the first_elt flag.
 xdisp.c (reseat_1): Initialize bidi_it.first_elt.  Move bidi
 scan start code from here...
 (next_element_from_buffer): ...to here.  Use bidi_it.first_elt flag.