Retrospective commit from 2009-10-05.

Retrospective commit from 2009-10-05.

Continue working on paragraph base direction.
Support per-buffer default paragraph direction.

 buffer.h (struct buffer): New member paragraph_direction.
 buffer.c (init_buffer_once): Initialize it.
 (syms_of_buffer): Declare Lisp variables
 default-paragraph-direction and paragraph-direction.
 dispextern.h (struct it): New member paragraph_embedding.
 xdisp.c (init_iterator): Initialize it from the buffer's value
 of paragraph-direction.
 <Qright_to_left, Qleft_to_right>: New variables.
 (syms_of_xdisp): Initialize and staticpro them.
 (set_iterator_to_next, next_element_from_buffer): Use the value of
 paragraph_embedding to determine the paragraph direction.
 bidi.c (bidi_line_init): Fix second argument to
 bidi_set_sor_type.
 (bidi_init_it): Initialize paragraph_dir to NEUTRAL_DIR.
 (bidi_get_next_char_visually): Record the last character of the
 separator in separator_limit, not the character after that.
 (bidi_find_paragraph_start): Accept character and byte positions
 instead of the whole iterator stricture.  All callers changed.