drupal_mkdir() doesn't work with absolute paths or recursive

When calling with an absolute path, such as '/Users/joachim/bin/drupal_hooks/8', this function returns FALSE.

This is because the call to explode produces an array like this:

When the directory path is then built up from the pieces, the first one will be an empty string.

For that matter, this looks wrong:

because the exploding on DIRECTORY_SEPARATOR means that none of the pieces contain DIRECTORY_SEPARATOR. Therefore, the joining will just smush the pieces together without slashes.