sd-dhcp-client: allways request broadcast

On systems which cannot receive unicast packets until its IP stack has been configured
we need to request broadcast packets. We are currently not able to reliably detect when
this is necessary, so set it unconditionally for now.

This is set on all packets, but the DHCP server will only broadcast the packets that are
necessary, and unicast the rest.

For more information please refer to this thread in CoreOS: https://github.com/coreos/bugs/issues/12

[tomegun: rephrased commit message]