Do not log errors for missing `pam_gnome_keyring.so`

[kimsin98]

Expected behaviour

If pam_gnome_keyring.so does not exist, silently ignore it. This can be achieved by using -auth in PAM configuration like lightdm (see pam.conf man pages).

Actual behaviour

mate-screensaver logs an error every time it is used because it tries to look for pam_gnome_keyring.so.

Steps to reproduce the behaviour

Install mate-screensaver without gnome-keyring

MATE general version

1.26

Package version

1.26.0

Linux Distribution

Arch Linux

Link to bugreport of your Distribution (requirement)

https://bugs.archlinux.org/task/72223

[zhuyaliang]

# Fedora Core
auth       include  system-auth
-auth       optional     pam_gnome_keyring.so