In the original InnoDB storage format (which was retroactively named
ROW_FORMAT=REDUNDANT in MySQL 5.0.3), the length of each index field
is stored explicitly. Thus, we can and from now on will allow arbitrary
extension of VARBINARY and VARCHAR columns when the table is in
ROW_FORMAT=REDUNDANT.

ha_innobase::open(): Advertise a new HA_EXTENDED_TYPES_CONVERSION
capability for ROW_FORMAT=REDUNDANT tables.

Field_varstring::is_equal(): If the HA_EXTENDED_TYPES_CONVERSION
capability is advertised for the table, return IS_EQUAL_PACK_LENGTH
for any length extension.