mac-learning: Refactor to increase generality.

mac-learning: Refactor to increase generality.

In an upcoming commit I want to store a pointer in MAC learning entries
in the bridge, instead of an integer port number.  The MAC learning library
has other clients, and the others do not gracefully fit this new model, so
in fact the data will have to become a union.  However, this does not fit
well with the current mac_learning API, since mac_learning_learn()
currently initializes and compares the data.  It seems better to break up
the API so that only the client has to know the data's format and how to
initialize it or compare it.  This commit makes this possible.

This commit doesn't change the type of the data stored in a MAC learning
entry yet.

As a side effect this commit has the benefit that clients that don't need
gratuitous ARP locking don't have to specify any policy for it at all.