ofproto-dpif: Add ovs-appctl commands for ovs-dpctl functions.

ofproto-dpif: Add ovs-appctl commands for ovs-dpctl functions.

These commands will be useful in a future commit that makes multiple
bridges share a single backing datapath.  The ovs-dpctl commands will
show information about the backing datapath, so it will be difficult to
determine which information belongs to which bridge.  The new "dpif/*"
ovs-appctl commands return information about the bridge--regardless of
how the backing datapath is configured.

Signed-off-by: Justin Pettit <jpettit@nicira.com>