Features:
+* Introduce $LISTEN_NAMES to complement $LISTEN_FDS, containing a
+ colon separated list of identifiers for the fds passed.
+
+* networkd: implement BindCarrier= logic to .network units that binds
+ application of the file to the carrier sense on another interface,
+ in order to implement uplink/downlink logic.
+
* make networkd subscribe to PrepareForSleep(false) and refresh all
DHCP leases then.