Module data is data associated with a module handle that is available
for the whole module loading time so it can be used also during
different operations.
We use cgo handles to preserve the life of the go objects so any value
can be associated with a pam transaction.
This allows to easily define go-handlers for module operations.
We need to expose few more types externally so that it's possible to
create the module transaction handler and return specific transaction
errors
