dpif: Add new functions dp_run() and dp_wait().

The upcoming netdev-based dpif needs a hook where it can process packets
and throw them against the flow table, and this provides a suitable place.

lib/dpif-linux.c

@@ -355,6 +355,8 @@ dpif_linux_recv_wait(struct dpif *dpif_)
 const struct dpif_class dpif_linux_class = {
     "",                         /* This is the default class. */
     "linux",
+    NULL,                       /* run */
+    NULL,                       /* wait */
     dpif_linux_open,
     dpif_linux_close,
     dpif_linux_delete,