sd-dhcp-client: use BPF on raw socket

Filter out everything except UDP packets destined for the DHCP client port,
this should avoid the vast majority of spurious wakeups.

Filter based on [0], with permission.

Possible improvemnts: also check for the DHCP magic cookie to drop invalid
packets. Check for our xid to filter out packets destined for other clients.

[0]: <https://github.com/ambrop72/badvpn/blob/master/dhcpclient/BDHCPClient.c#L57>