DOWNGRADED FROM 5.6.11 TO 5.6.10

Problem was new syntax not accepted by previous version.

Fixed by adding version comment of /*!50531 around the
new syntax.

Like this in the .frm file:
'PARTITION BY KEY /*!50611 ALGORITHM = 2 */ () PARTITIONS 3'
and also changing the output from SHOW CREATE TABLE to:
CREATE TABLE t1 (a INT)
/*!50100 PARTITION BY KEY */ /*!50611 ALGORITHM = 1 */ /*!50100 ()
PARTITIONS 3 */

It will always add the ALGORITHM into the .frm for KEY [sub]partitioned
tables, but for SHOW CREATE TABLE it will only add it in case it is the non
default ALGORITHM = 1.

Also notice that for 5.5, it will say /*!50531 instead of /*!50611, which
will make upgrade from 5.5 > 5.5.31 to 5.6 < 5.6.11 fail!
If one downgrades an fixed version to the same major version (5.5 or 5.6) the
bug 14521864 will be visible again, but unless the .frm is updated, it will
work again when upgrading again.

Also fixed so that the .frm does not get updated version
if a single partition check passes.