This allows the authentication modules to perform SASL proxy authentication. It puts the onus on them to authorize the authcid to masquerade as the authzid. Doesn't currently implement such functionality in existing auth modules, since they cannot currently codify a relationship between the two identities. Does not permit the authzid to use a domain differently from the one of the connection.
Note: digest might not work, but I have no interest in it, being deprecated.
