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.

lib-master: Added timeout for auth master lookups. --HG-- branch : HEAD

Added support for userdb lookup to fail with a reason (many API changes). --HG-- branch : HEAD

imap, pop3: Close auth/config connections when we stop accepting more connections. --HG-- branch : HEAD

lib-master: Fixes to handling master-login services. --HG-- branch : HEAD

Redesigned how login process passes connections to mail processes and changed related APIs. Master process is no longer in the middle. --HG-- branch : HEAD