Make it possible to specify engine-defined attributes on partitions
as well as tables.

If an engine-defined attribute is only specified at the table level,
it applies to all the partitions in the table.
This is a backward-compatible behavior.

If the same attribute is specified both at the table level and the
partition level, the per-partition one takes precedence.
So, we can consider per-table attributes as default values.

One cannot specify engine-defined attributes on subpartitions.

Implementation details:

* We store per-partition attributes in the partition_element class
  because we already have the part_comment field, which is for
  per-partition comments.

* In the case of ALTER TABLE statements, the partition_elements in
  table->part_info is set up by mysql_unpack_partition().
  So, we parse per-partition attributes after the call of the function.