Alterações na distribuição 3.21.25

Changed typedef 'string' to typedef 'my_string' for better portability.

You can now kill threads that are waiting on a disk-full condition.
Fixed some problems with UDF functions.
Added long options to isamchk. Try isamchk --help.
Fixed a bug when using 8 bytes long (alpha); filesort() didn't work. Affects DISTINCT, ORDER BY and GROUP BY on 64-bit processors.