lib-storage: mailbox_exists() now returns namespace prefix as selectable if it is.
lib-storage: mailbox_exists() now returns namespace prefix as selectable if it is.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot