lib-storage: Change mail_user_autoexpunge() to return number of autoexpunged mails

Added mailbox { autoexpunge } setting. This can be used to automatically expunge mails from specified mailboxes after they're old enough. The expunges are done when the user is being deinitialized. mailbox_list_index=yes should be enabled to have the best performance with this setting. Example: namespace inbox { mailbox Spam { auto = create special_use = \Junk autoexpunge = 30d } }