(Qshow_trailing_whitespace): Removed.

(Qshow_trailing_whitespace): Removed.
(Vshow_trailing_whitespace): Added.
(init_iterator): Remove initialization code for
show_trailing_whitespace_p.
(redisplay_internal): Don't try cursor movement in this_line
if showing trailing whitespace.
(redisplay_window): Likewise for cursor movement in current
matrix and try_window_id.
(try_window_reusing_current_matrix): Likewise.
(trailing_whitespace_p): Return 0 if trailing whitespace is
in front of point.
(display_line): Use Vshow_trailing_whitespace instead of
former iterator member show_trailing_whitespace_p.
(syms_of_xdisp): Add DEFVAR_LISP for show-trailing-whitespace.