Pass a pointer to user_env in to the init_session policy plugin

Pass a pointer to user_env in to the init_session policy plugin
function so session setup can modify the user environment as needed.
For PAM authentication, merge the PAM environment with the user
environment at init_session time.  We no longer need to swap in the
user_env for environ during session init, nor do we need to disable
the env hooks at init_session time.