Arvados

I think the way this is supposed to work:

1. pam_sm_authenticate() calls pam_set_data() to stash the Arvados token
2. pam_sm_open_session() uses pam_get_data() to read the token back out
3. pam_sm_open_session() calls pam_putenv() to set ARVADOS_API_TOKEN and ARVADOS_API_HOST.