Implement SocketUser= and SocketGroup= for [Socket]

Since we already allow defining the mode of AF_UNIX sockets and FIFO, it
makes sense to also allow specific user/group ownership of the socket
file for restricting access.

src/shared/socket-label.c

@@ -52,6 +52,7 @@ int socket_address_listen(
                 int *ret) {
 
         int r, fd, one;
+
         assert(a);
         assert(ret);