Add pam configuration to allow user sessions to work out of the box

Add pam configuration to allow user sessions to work out of the box

systemd-logind will start user@.service. user@.service unit uses
PAM with service name 'systemd-user' to perform account and session
managment tasks. Previously, the name was 'systemd-shared', it is
now changed to 'systemd-user'.

Most PAM installations use one common setup for different callers.
Based on a quick poll, distributions fall into two camps: those that
have system-auth (Redhat, Fedora, CentOS, Arch, Gentoo, Mageia,
Mandriva), and those that have common-auth (Debian, Ubuntu, OpenSUSE).
Distributions that have system-auth have just one configuration file
that contains auth, password, account, and session blocks, and
distributions that have common-auth also have common-session,
common-password, and common-account. It is thus impossible to use one
configuration file which would work for everybody. systemd-user now
refers to system-auth, because it seems that the approach with one
file is more popular and also easier, so let's follow that.