datapaths: Restructure action execution

Action handlers are now bound at initialization time based on
their names, similar to incoming message handlers.  We
should add a way to do something similar for vendor actions.
datapaths: Don't add disabled action handlers

If an action type is disabled in the SwitchFeatures object of a
switch, don't add the corresponding handler to the handler table.
datapaths: Generate ofp_error on bad action type

Previously, a missing action type raised an exception, but this
isn't really an exceptional condition.  We now just log the
fact that it happend and send the controller an appropriate
ofp_error.