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

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.