mail and Maildir subdirectories created when unnecessary

There really are two related issues here. The first is simple: The mail subdirectory is still automatically created for each new user account, and this directory is no longer used since Virtualmin Pro currently mandates (or at least only supports) the use of Maildir.

The second issue is that the Maildir directory is created for every new user, regardless if they have e-mail enabled or not. It would make more sense to not create this directory unless and until the user has a primary e-mail account enabled.