cmp_data(): Compare different-length CHAR fields with
the new strnncollsp_nchars function that will pad spaces if needed.

Any InnoDB ROW_FORMAT except the original one that was named
ROW_FORMAT=REDUNDANT in MySQL 5.0.3 will internally store
CHAR(n) columns as variable-length if the character encoding is
variable length. Spaces may be trimmed from the end.
For NOT NULL values, the minimum length is always n*mbminlen.
In cmp_data() we only know the lengths in bytes and we cannot
easily know the ROW_FORMAT.

is_strnncoll_compatible(): Refactored from innobase_mysql_cmp().

innobase_mysql_cmp(): Merged to cmp_whole_field().

cmp_whole_field(): Invoke strnncollsp_nchars for the DATA_MYSQL
(the CHAR type with any other collation than latin1_swedish_ci).

Reviewed by: Alexander Barkov
Tested by: Roel Roel Van de Paar