Modules have the ability to start PAM conversations, so while the
transaction code can handle them we did not have a way to init them.
Yet.
So add some APIs allowing this, making it easier from the go side to
handle the conversations.
In this commit we only support text-based conversations, but code is
designed with the idea of supporting binary cases too.
Added the integration tests using the module that is now able to both
start conversation and handle them using Go only.
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 will make it easier to avoid exporting unexpected symbols to the
generated PAM libraries.
Also it makes less messy handling C code inside go files.
