add a small script in /etc/profile.d/dbus.sh to export DBUS_SESSION_BUS_ADDRESS

add a small script in /etc/profile.d/dbus.sh to export DBUS_SESSION_BUS_ADDRESS

If the dbus session address is not defined, this small script will try to
pick the address from the environment of the systemd --user process for the current
user.

Typically, this allows to log on a target through ssh, then run su - <user> and
have the same environment as the shells inside the user session.

Change-Id: Id6133077bf9943c4203f7d993b8942dc1455bef5
Signed-off-by: Stephane Desneux <stephane.desneux@open.eurogiciel.org>