The caller doesn't care which plugin is being used, so this commit
consolidates implementation details within auth.js
The motivation for this work is to prepare for extending the auth API
(to allow "advanced" LDAP to query user entry ontological state at start
up), by tidying up rather than duplicating the existing mechanism.
Also draft some kind of plugin system for auth, although it essentially consists in writing a function
and there is no mechanism to automatically fallback from one auth to another
