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

src/network/org.freedesktop.network1.service

@@ -0,0 +1,12 @@
+#  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.
+
+[D-BUS Service]
+Name=org.freedesktop.network1
+Exec=/bin/false
+User=root
+SystemdService=dbus-org.freedesktop.network1.service