- Fix try_first_pass / use_first_pass (pam_get_authtok() code &
  documentation are slightly incorrect, OpenPAM's pam_unix(8) is
  incorrect, all FreeBSD modules are broken)

- Add loop detection to openpam_load_chain().

- Complete unit tests for openpam_dispatch().

- Stop using PAM_SYMBOL_ERR incorrectly.
