(current_column_bol_cache): New variable. This makes

(current_column_bol_cache): New variable.  This makes
move_to_column faster.
(current_column): Set current_column_bol_cache.
(current_column_1): Likewise.
(Fmove_to_column): Use current_column_bol_cache.

(current_column): When the buffer may have multi-byte
characters, call current_column_1.
(compute_motion): Handle multi-byte characters.
(current_column_1): Likewise.
(Fmove_to_column): Likewise.

Include "charset.h".
(compute_motion): Remove the variable PREV_VPOS.
Introduce new local variables (multibyte, wide_column, prev_pos,
contin_hpos, prev_tab_offset) to handle multi-byte characters and
wide-column characters.

(pos_tab_offset): Specify "- (1 << (BITS_PER_SHORT - 1))"
to express big negative number (for consistency with xdisp.c).

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