pam_ssh_agent_auth: Re-allow multiple authorized keys files

This functionality was initially introduced in
3644f912 to fix
https://github.com/NixOS/nixos/pull/52, but was broken in the update from 0.9.5
to 0.10.3.  The original patch does not cleanly apply due to reformatting and
parameter changes upstream, but the adaptations of the patch to the new version
are not too severe.