Client API
----------
Basically the Authen::SASL module gathers some info. When ->client_new
is called the plugin is called to create a $conn object. At that point
it should query the Authen::SASL object for mechanisms and callbacks

Properties are then set on the $conn object by calling $conn->property

Then client_start is called

Then we call client_step with a challenge string to get a response
string. need_step can be called to check that this step is actually
necessary for the selected mechanism.


Quite simple really I think.


So the plugin just needs to support

  client_new
  client_start
  client_step
  need_step    # returns true if client_step needs to be called
  property     # set/get for properties
  mechanism    # returns the name of the chosen mechanism
  service      # the service name passed to client_new
  host         # the hostname passed to client_new
  is_success   # returns true if authentication suceeded

Server API
----------
The server API is symetric to the client's one. server_new is called to
create a connection object. Then server_start is called, and if relevant
the first data from the client is passed to it as argument.

Then we call server_step with all the response from the clients, which
returns challenges. need_step also determines if the current mechanism
requires another step.

So the plugin just needs to support

  server_new
  server_start
  server_step
  need_step    # returns true if client_step needs to be called
  property     # set/get for properties
  mechanism    # returns the name of the chosen mechanism
  service      # the service name passed to client_new
  host         # the hostname passed to client_new
  is_success   # returns true if authentication suceeded

Callbacks
---------
properties and callbacks are passed by name, so you will need to convert
them to numbers.

There are three types of call back

  user => 'fred'

When the user callback is called, it will just return the string 'fred'

  user => \&subname

When the user callback is called, &subname will be called and it will
be passed the $conn object as the first argument.

  user => [ \&subname, 1, 2, 3]

When the user callback is called, &subname will be called. It will be passed
the $conn object, followed by all other values in the array