Fix for BUG#4375 "Windows specific directories are copied during
replication": in the Windows version of my_dir(), do not show hidden
or system files which Windows sometimes creates in the root directory
of drive (like RECYCLER and SYSTEM VOLUME INFORMATION directories) (so
this problem showed up only when the datadir was the root dir of a
drive).
This will make SHOW DATABASES display better, and will do no harm to
MySQL. For example, DROP DATABASE will not miss some files, as MySQL
creates no hidden or system files.