Handle syncing of mailboxes with \Noselect folders

Shared mailbox folders are often created within a hierarchy that includes "Shared" or the like dummy folder. The folder can have the IMAP attribute \Noselect set, to avoid clients reading or writing emails there. The current imap backend does not honour the \NoSelect attribute. 

 

This change introduces a "dontstat" array to keep track of imap folders which should not be synced. This prevents errors within z-push and causes any client-directed email moves to non-selectable folders to fail.