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.