attributes with a pointer to the unix_user_db object.
(pwhelper_callback): New class.
(do_pwhelper_callback): New function.
(make_pwhelper_callback): Likewise.
(kerberos_check_pw): Made non-blocking.
(do_verify_password): Try ordinary unix passwords first, and use
pw_helper only if ordinary password fails.
(unix_user_db): Added reaper attribute.

Rev: src/unix_user.c:1.27