Isync

IMAP provides a way to specify accessing Mailboxes as a different user
then the user used for authentication.  Currently isync is missing this
feature, which is present for example in imapsync.

The User keyword was being used both for authentication and
authorization.  Now a new keyword Author is introduced, which is used as
the authorization id if given.  The User keyword is used for
authentication, because the User/Password combination seams to be
natural for login credentials.  If Author isn't specified the value of
User is taken as was always the case.  Note, that this differs from the
behaviour described in RFC4616, where AUTHZID is supposed to be empty if
it is to be derived from AUTHCID.
However, in 8dc776c5 it is mentioned,
that this causes problems with the GSS-API module adding a REALM, so it
seams safer to keep that non-standard behaviour for now.