Changes according to Davi's review.

Implemented EXCHANGE PARTITION as a Sql_statement
object, and also moved the generic ALTER TABLE
to a Sql_statement.

Moved code from sql_table.{h,cc} into
sql_alter_table.{h,cc} and sql_partition_admin.{h,cc}

Fixed list_files in mysqltest to allow replace_result.
(fix by Bjorn Munch)