bump(dev-libs/libdnet): initial import

Needed by open-vm-tools, which is only installed when a VMware OEM image
is generated.

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