per-user dbus

[Havoc Pennington]

If you really want to melt your brain on this topic, realize that it
basically requires some decisions on that "what does su do" bug. I
still think certain cases will have to be proven impossible / declared
unsupported to close that one.

The big reason for a session bus, not (user,machine) bus historically,
was that old-KDE had everything per-session and largely Just Worked
with NFS and multiple sessions, while old-GNOME was trying to do
various stuff per-(user,machine) - or some really broken hacks trying
for true per-user - and was breaking horribly with multiple sessions.

If developers don't really think about multiple logins or NFS,
per-session tends to mostly work as expected, while per-(user,machine)
tends to get really hosed because a program has to be aware of
multiple X servers and sessions to work properly if it's scoped
per-(user,machine). Unfortunately, if there's something called
DBUS_BUS_USER I think lots of devs will think "oh, I should use that"
for something like config settings. But in all likelihood, they should
not use it, because it will just fail - it would only "magically work"
in the right way if it were per-user, not per-(user,machine).

per-(user,machine) is pretty much only useful for hardware-related
stuff. Otherwise it needs to be per-user, not per-(user,machine).

Havoc