losing it's .MYD

When OPTIMIZE TABLE is completed it attempts to rename temporary
file to original name. This step may fail on windows when a file
is opened. As a result data file might be deleted and optimized
copy of file (table_name.MYD) remains.

This situation is handled properly by my_delete_allow_opened, so
use it instead of my_delete when attempting to rename a file on
windows.

No suitable test case for this bug.