* search.c (find_newline): Accept start and end byte positions

* search.c (find_newline): Accept start and end byte positions
as arguments and allow -1 if not known.
(find_newline_no_quit): Likewise for start position.
* lisp.h (find_newline, find_newline_no_quit): Adjust prototype.
* bidi.c (bidi_find_paragraph_start): Pass byte position to
find_newline_no_quit, thus eliminating CHAR_TO_BYTE.
* editfns.c (Fconstrain_to_field): Break long line.  Adjust
call to find_newline.
* indent.c (vmotion): Adjust calls to find_newline_no_quit.
Use DEC_BOTH to start next search from the previous buffer
position, where appropriate.
* xdisp.c (back_to_previous_line_start, forward_to_next_line_start)
(get_visually_first_element, move_it_vertically_backward): Likewise.
Obtain byte position from the display iterator, where appropriate.