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

  Currently client_start returns the mechanism name and the initial
  string, but I am thinking about changing that to just the initial
  string. The mecanism is avaliabe via a method call anyway.


Then we call client_step with a challenge string to get a response
string.


Quite simple really I think.


So the plugin just needs to support

  client_new
  client_start
  client_step
  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


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
