networkd: add basic dbus API

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