per-user dbus

[Havoc Pennington]

And again, if you do add a user bus, you really really want it to be
DBUS_BUS_USER_KERNEL_PAIR or something very explicit, so devs are
forced to stop and think "oh, what does that mean and what are the
implications..." while with DBUS_BUS_USER they might just think
"perfect!" and try using that for everything under the sun when it's
basically not very useful and should not be used in most cases.

Havoc