login: support user-bus on dbus1

dbus-1.10 was just released, including systemd units to run
`dbus-daemon --session` as systemd user unit. This allows using a
user-bus with dbus1, just like we do per default with kdbus.

All the dbus libraries have already been fixed long ago to use the
user-bus as default. Hence, there's no need to set
DBUS_SESSION_BUS_ADDRESS= if we use the user-bus. However, gdm and
friends continue to spawn a session bus if this variable is not set
(instead of checking for the existence of the user-bus). Hence, we force
the user-bus, if it is available, in pam_systemd. Once gdm and friends
are fixed, we can continue to drop this again. However, that might take
a while.

With this in place, all that is needed to make the user-bus work is:
    `systemctl --global enable dbus.socket`

If dbus.socket is not enabled, the legacy session-bus is still used.

Based on a patch by: Jan Alexander Steffens <jan.steffens@gmail.com>

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