Made PKG_STATEDIR configurable with state_dir setting. Based on patch by Chris Webb. Normally this isn't needed, since the data in the state_dir can be shared across multiple Dovecot instances, but in some specific use cases this may be useful (e.g. users running their own Dovecots).

lib-master: Keep track of config paths in "instances" file. Normally it can be looked up from base_dir/dovecot.conf symlink, but base_dir may get deleted when system starts up.

lib-master: Removed code to auto-drop instances from the list. There couldn't be any empty named instances, so it was a bit pointless.

Keep track of what Dovecot instances have been executed and their instance_name. doveadm instance command can be used to list/remove them.