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 systemd 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 systemd socket activation.

src/basic/socket-util.h

@@ -30,6 +30,7 @@
 #include <linux/if_packet.h>
 
 #include "macro.h"
+#include "missing.h"
 #include "util.h"
 
 union sockaddr_union {
@@ -40,6 +41,7 @@ union sockaddr_union {
         struct sockaddr_nl nl;
         struct sockaddr_storage storage;
         struct sockaddr_ll ll;
+        struct sockaddr_vm vm;
 };
 
 typedef struct SocketAddress {