(display_text_line): Introduce new local variable

(display_text_line): Introduce new local variable
rev_dir_bit to display right to left characters (not yet used).

(message): Use FRAME_MESSAGE_BUF_SIZE().
(redisplay_internal): Add canceling code for continuation at
wide-column.
(display_text_line): Don't just decrement left_edge->bufpos,
it may be multi-byte character, use DEC_POS instead.

(try_window): Change the way of calculation of tab
offset.  We now use val.tab_offset to maintain tab offset.
Removed local variable tab_offset.
(try_window_id): Likewise.

(pos_tab_offset): Return COL (Modulo is no longer valid).
Add the line to set tab_offset member.
Use pos.tab_offset.

(redisplay_window): Specify big negative number for TOHPOS
of compute_motion().
(try_window_id): Likewise.

(tri_window_id): Specify "1 << (BITS_PER_SHORT - 1)" to
express "Don't care".

Include charset.h, coding.h, and process.h.
(display_text_line): Handle multibyte characters.
(display_mode_line): Pay attention to wide-column characters.
(decode_mode_spec_coding): New function.
(decode_mode_spec): Handle %-constructs `%z' and `%Z' to print
coding system mnemonics.
(display_string): Handle multibyte characters.