class type auth_method =object..end
An auth_method object represents a method of authentication. Such an
 object can be shared by several clients.
method name : stringThe name of this method, used for errors etc.
method new_session : t -> string option -> auth_protocolRequest a new session. The 2nd argument is the user name, or None
	     if the default is to be used (whatever this is). Some
	     authenticators only support None.
It is allowed that the returned auth_protocol object is already
	     in state `Done, i.e. that actually no protocol is run.
Changed in Ocamlnet-3.3: different signature. The user name is
	     now an argument, and the method returns auth_protocol instead
	     of auth_session. There can now be a separate session for
	     each user (plus for the default user None).