This commit makes pam_fprintd return PAM_UNKNOWN_USER when
the user has not enrolled a fingerprint.

This lets the administrator set up pam_fprintd as a required
authentication, method, but only for users that have enrolled a
fingerprint, as such:

auth  [success=ok user_unknown=ignore default=die] pam_fprintd.so max_tries=1 timeout=-1
auth  [success=1  default=ignore]                  pam_unix.so nullok_secure
auth  requisite                                    pam_deny.so

With this config, users w/o an enrolled fingerprint will just be
asked for a password.  Users with an enrolled fingerprint will
required to login using both their fingerprint and a password.

https://bugs.freedesktop.org/show_bug.cgi?id=64781