autolaunch uuid support

[Havoc Pennington]

Havoc Pennington wrote:
>  - an implied option --use-existing means try to read an existing bus
>    from in order 1) DBUS_SESSION_BUS_ADDRESS 2) the display
>    and prefer these to launching a new bus. libdbus
>    may need to unset DBUS_SESSION_BUS_ADDRESS before
>    exec'ing dbus-launch.
> 

Something that may not be clear here is that e.g.

  dbus-launch --use-existing --no-launch

is supposed to work, this would read any existing bus address and print 
it out, without starting a new bus.

Havoc