socket-util: add AF_VSOCK address family

socket-util: add AF_VSOCK address family

The AF_VSOCK address family facilitates guest<->host communication on
VMware and KVM (virtio-vsock).  Adding support to elogind allows guest
agents to be launched through .socket unit files.  Today guest agents
are stand-alone daemons running inside guests that do not take advantage
of elogind socket activation.