WL#2629: SHOW TABLE STATUS: Include InnoDB index info

InnoDB computes the sum of index sizes in ha_innobase::info(), so that SHOW
TABLE STATUS can show the space consumption by index.

And ...

In INFORMATION_SCHEMA.TABLE_COMMENT, the amount of free space
is appended to the regular comment. For example:

mysql> create table tmo (s1 int) comment '!!' engine=innodb;
Query OK, 0 rows affected (0.01 sec)

mysql> select table_comment from information_schema.tables where table_name = 'tmo';
+--------------------------+
| table_comment            |
+--------------------------+
| !!; InnoDB free: 4096 kB |
+--------------------------+
1 row in set (0.00 sec)

This information could appear instead in SHOW TABLE STATUS,
and in INFORMATION_SCHEMA.TABLES, as a separate column.
Gluh has suggested the name EXTRA for this column.

See also BUG#11379.