Added support to perform token-based service process authentication. Creates hidden SASL method DOVECOT-TOKEN. This method is not available on the normal auth login socket and thus never presented to clients. Creates new auth socket type 'tokenlogin'. This otherwise normal login socket only offers authentication using the DOVECOT-TOKEN mechanism. Creates new token-login directory in base_dir to separate token logins from normal logins. This directory is otherwise completely identical to the normal login dir, i.e. it contains sockets for the service backends, used to chroot login processes to, etc. Makes default login socket configurable. Performs some minor changes to src/login-common to build very sparse protocols, e.g. avoid the need to implement methods that are not needed.