Return PAM_IGNORE from pam_sm_authenticate

Summary:
PAM_SUCCESS signals to the PAM stack that the
authentication succeeded. That is actually not the case, because
pam_kwallet doesn't authenticate anything. If a user would badly configure
his PAM stack by classifying pam_kwallet as sufficient then login would
always be possible.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: mgerstner, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12910