Allow support for mailboxes outside INBOX

Case 121565: Allow support for mailboxes outside of the INBOX prefix, rename
Maildir++ support flag

Changes:

    * Make Mail::Dir->open() accept 'maildir++' flag instead of the old,
      ambiguous 'with_extensions' flag

    * Implement internal function validate_mailbox_name() as central means
      of enforcing some modicum of mailbox name sanity

    * Modify $maildir->mailbox_dir() to allow mailboxes outside the INBOX
      prefix

    * Modify $maildir->mailbox_dir() to return base directory of Maildir++
      hierarchy when passed INBOX

    * Update unit tests to ensure $maildir->mailbox_dir() mapping acts in
      a desirable manner