Implement credentials caching in pam responder.
Currently works only for the proxy backend.
Also cleanup pam responder code and mode common code in data provider.
(the data provider should never include responder private headers)

The same module may implement both types, but initializatrion will be
nonetheless performed separately, once for the identity module and once for the
authenticator module.

Also change the proxy module to retireve the pam target name from the domain
configuration so that it is possibile to create per-domain pam stacks.

With this modification it is actually possibile to use normal nss and pam
modules to perform a successful authentication (tested only with sudo so far)

Update exmples.