Allow more than one callback to be register to NetworkInterfaces.
This introduces new APIs:
void NetworkInterface::add_event_listener(...);
void NetworkInterface::remove_event_listener(...);

Which internally calls interfaces attach() functions.

->move() operator was not touching unused data fields, therefore
leaving uninitialised data and failing the comparison.
Fixed by initialising all fields to zero before moving.

Standard new operator already calls MBED_ERROR in failure.

…aries