Adds open-vm-tools, libdnet and oem-vmware packages

This PR continues on the work started by @crawford on porting open-vm-tools. Additionally, it adds the oem-vmware ebuild and a coreos-init change to add a SystemD network unit in order to enable receiving DHCP offers through broadcast, for VMware images **only**.

Except for the systemd network unit added in coreos-init, these three packages and their dependencies are only installed on VMware OEM images.

This PR is part of a larger change-set intended to create a good starting point for VMware OEM images. The change-set is composed of the following PRs: https://github.com/coreos/scripts/pull/336, https://github.com/coreos/coreos-overlay/pull/898, https://github.com/coreos/init/pull/137, https://github.com/coreos/portage-stable/pull/139
What does this dependency look like? I'm reluctant to have OEM packages link against libraries in the base OS other than glibc since the OEM partition is not upgraded along with /usr right now. I suppose glib does maintain good ABI compatibility so making an exception for it may be OK.