dpif: Improve abstraction by making 'run' and 'wait' functions per-dpif.

Until now, the dp_run() and dp_wait() functions had to be called at the top
level of the program because they applied to every open dpif.  By replacing
them by functions that take a specific dpif as an argument, we can call
them only from ofproto, which is currently the correct layer to deal with
dpifs.

lib/dpif-linux.c

@@ -994,12 +994,12 @@ dpif_linux_recv_purge(struct dpif *dpif_)
 
 const struct dpif_class dpif_linux_class = {
     "system",
-    NULL,                       /* run */
-    NULL,                       /* wait */
     dpif_linux_enumerate,
     dpif_linux_open,
     dpif_linux_close,
     dpif_linux_destroy,
+    NULL,                       /* run */
+    NULL,                       /* wait */
     dpif_linux_get_stats,
     dpif_linux_get_drop_frags,
     dpif_linux_set_drop_frags,