sd-dhcp-client: Sets broadcast flag when driver does not support layer2

VMware VMs using bridged networking are unable to receive DHCPOFFER
messages through unicast. Drivers tested are vmxnet3 and e1000.

More information about this issue can be found at the following places:

- Issue originally reported at https://github.com/coreos/bugs/issues/12
- Mailing list discussion: http://lists.freedesktop.org/archives/systemd-devel/2014-May/019560.html

[tomegun: made the feature conditional]