networkd: add basic dbus API

Only the very basics, more to come.

For now:

$ busctl tree org.freedesktop.network1
└─/org/freedesktop/network1
  └─/org/freedesktop/network1/link
    ├─/org/freedesktop/network1/link/1
    ├─/org/freedesktop/network1/link/2
    ├─/org/freedesktop/network1/link/3
    ├─/org/freedesktop/network1/link/4
    ├─/org/freedesktop/network1/link/5
    ├─/org/freedesktop/network1/link/6
    ├─/org/freedesktop/network1/link/7
    ├─/org/freedesktop/network1/link/8
    └─/org/freedesktop/network1/link/9

$ busctl introspect org.freedesktop.network1 /org/freedesktop/network1
NAME                                TYPE      SIGNATURE RESULT/VALUE FLAGS
org.freedesktop.network1.Manager    interface -         -            -
.OperationalState                   property  s         "carrier" emits-change

$ busctl introspect org.freedesktop.network1 /org/freedesktop/network1/link/1
NAME                                TYPE      SIGNATURE RESULT/VALUE FLAGS
org.freedesktop.network1.Link       interface -         -            -
.AdministrativeState                property  s         "unmanaged" emits-change
.OperationalState                   property  s         "carrier" emits-change

units/org.freedesktop.network1.busname

@@ -0,0 +1,14 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Network Service Bus Name
+Documentation=man:systemd-networkd.service(8)
+
+[BusName]
+Service=systemd-networkd.service
+AllowWorld=talk