handle messages without DHCP options

(e.g. plain BOOTP replies without additional information)

Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>
added sd_notify() support

Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>
trigger systemd's watchdog

Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>
test release 0.11.90

Signed-off-by: Enrico Scholz <enrico.scholz@ensc.de>