WL#4651: Backup: add backup driver for partitioned MyISAM tables

When a partitioned MyISAM table is backed up, backup has to use the Default
driver. This WL is for adding a backup driver for partitioned MyISAM tables.

Rationale:
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When backing up a MyISAM table, the native MyISAM driver is preferred to the
Default driver. The reason is that the default driver blocks operations on the
table for a much longer period than the native driver does. 

Note that the Snapshot driver can be used for partitioned Falcon and InnoDB
tables, so this problem is only related to partitioned MyISAM tables.

Why we need a partitioned driver:
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The current native driver cannot handle partitioned MyISAM tables because MyISAM
tables are stored in files called .MYI whereas partitioned MyISAM
tables are stored in files called .MYI. 

Handling of partitioned tables can be solved in multiple ways:

1. Modify the existing MyISAM native driver to handle partitioned tables
2. Add a new 'Partitioned MyISAM' driver. The new partitioned driver would lend
a lot of functionality from the existing native driver
3. Add a generic partitioned driver. It will be used for any partitioned table
and will make use of the most appropriate driver for each of the partitions.