RSPAN does not work properly unless MAC learning for the VLAN is
disabled on all switches between the origin and monitoring point.
This allows learning to be disabled on a given VLAN so vSwitch can
acts as an intermediate switch.
Feature #2136
The vswitchd bonding code needs to iterate through the table entries to
be able to send out gratuitous learning packets when bond slaves go down.
It might be best to create an abstract interface to the MAC learning table,
but this commit does the simpler thing and exposes the data structures in
the header file.
