Choose another way to fix the issue from r5804: When parsing the directory

Choose another way to fix the issue from r5804: When parsing the directory
name, add a file mask, so the result is "$0:*" or similar. When specified like
this, the drive accepts the r/o specifier ",R" so we don't have to complicate
the code in open().

git-svn-id: svn://svn.cc65.org/cc65/trunk@5805 b7a2c559-68d2-44c3-8de9-860c34a00d81