manager: Lock down the communication between the daemons

This is done by starting an additional dbus daemon on a private socket, then
passing an fd handle to the user process. Requesting a handle is validated to
ensure that the caller is the user daemon, otherwise it returns an error.

data/root-dbus.conf

@@ -0,0 +1,18 @@
+<!DOCTYPE busconfig PUBLIC "-//freedesktop//DTD D-Bus Bus Configuration 1.0//EN"
+ "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
+<busconfig>
+  <type>session</type>
+  <keep_umask/>
+  <listen>unix:tmpdir=/var/run/steamos-manager</listen>
+  <auth>EXTERNAL</auth>
+  <auth>ANONYMOUS</auth>
+  <allow_anonymous/>
+  <policy context="default">
+    <allow send_destination="*" eavesdrop="true"/>
+    <allow eavesdrop="true"/>
+    <deny own="*"/>
+  </policy>
+  <policy user="root">
+    <allow own="*"/>
+  </policy>
+</busconfig>