WL#3525: Allow MyISAM to be used with Direct I/O

Allow MyISAM tables to be used with Direct I/O.

Currently, Direct I/O can only be used with InnoDB (--forcedirectio option):

http://dev.mysql.com/doc/refman/5.0/en/innodb-tuning.html

I realize that this cannot be done with MyISAM currently because MySQL does not
cache data files.  However, if this is something that can be done in the future,
that would be very helpful for those that have Direct I/O.

I am unsure of the feasibility of this request, so please let me know your
comments.