Kdbus meets linux-kernel

At a fundamental level it is not needed. In fact, we have dbus implementation that are not in the kernel. All that is needed to implement such a userspace dbus is a minimal way to connect to a socket and send bits (and optionally pass fd:s).

However, being in the kernel has certain advantages:

* Routing can use less context switches
The non-kernel dbus has to do the routing in a daemon process that all
clients connect to. This means any message needs to be sent first to the
daemon and then to the target process. In the kernel we can route direct from source to target process.

* Less copying of messages
As per the above, we need to copy and parse the message into the daemon
process first, and then into the destination.

* Security verification can be done in the kernel
Atm the dbus daemon applies things like uid limits and selinux policy on
dbus messages. If this is done in the kernel there is a higher level of
trust and auditing of this.

There has been multiple tries to make dbus work in the kernel. Some have involved modifying AF_UNIX or making a new network domain that have some of the dbus semantics. However, they have all been shot down by the network subsystem maintainer with permanent NACKs. This is why this new version uses device nodes instead.