dhcp-network: don't pass ifindex to bind_udp_socket

UDP sockets can anyway not be bound to specific netdev's. The packages would have to be filtered
when received instead.
sd-dhcp: network - set TOS on outgoing packets

This should improve performance on busy wireless networks and the
like. Inspired by a similar change in dnsmasq.
sd-dhcp: network - don't leak sockets on failure

Also some general cleanups