daemon: import DISPLAY into user bus

commit 99eeae91 attempted to
support dbus user buses with X sessions, by supposedly importing
DISPLAY and XAUTHORITY into the dbus user bus activation
environment.

It didn't actually work, though, because the spawn_bus function
quits early if the dbus daemon is already running.

This commit fixes the code so that the activation environment is
always updated.

https://bugzilla.gnome.org/show_bug.cgi?id=761568