[patch] direct service connections

[Colin Walters]

Thinking about this a bit more, let's just punt on this for now if it
would require major core changes.  I only really envision two users of
Imsep (gdk-pixbuf and libpr0n), so a bit more code in them won't really
hurt much.  Imsep is probably a bit unique with respect to its
amount/size of traffic too.  But if there is anyone else doing high-
traffic IPC over D-BUS too, we can revisit this patch later.

Although I am thinking now it would still be useful to have
DBUS_BUS_SESSION_LOCAL, so Imsep doesn't become insanely slow on thin
client setups.