LOAD DATA into partitioned MyISAM table

Problem was that both partitioning and myisam
used the same table_share->mutex for different protections
(auto inc and repair).

Solved by adding a specific mutex for the partitioning
auto_increment.

Also adding destroying the ha_data structure in
free_table_share (which is to be propagated
into 5.5).

This is a 5.1 ONLY patch, already fixed in 5.5+.