This patch changes the main name of 3 byte character set from utf8 to
utf8mb3. New old_mode UTF8_IS_UTF8MB3 is added and set TRUE by default,
so that utf8 would mean utf8mb3. If not set, utf8 would mean utf8mb4.
Two bugs in Aria, related to 2-level fulltext indexes:
* REPAIR calculated the key number incorrectly
* CHECK copied the key into last_key too early and
checking the second-level btree was overwriting it