file_check_directory() does not created nested directories

When calling file_check_directory() to ensure that a directory exists, the specified directory will be created if it is not found. This works fine if all the parent directories on the path exist, but breaks if they don't.

Example:
file_check_directory('/www/files/xxx/yyy', FILE_CREATE_DIRECTORY)
will succeed if /www/files/xxx exists, but fail if only /www/files exists.

This can be corrected for PHP5 and above users by using the $recursive flag when mkdir is called. If PHP 4 is detected, the path would need to be built a level at a time, from a loop.

This fix would also require passing a $mode argument to mkdir, which would need to be also added to file_check_directory().