Use expiration field to indicate registration/unregistration in registration callback

[ming]

In registration callback on_reg_state2(), especially in failure cases, sometimes it's difficult to distinguish whether the callback is called for registration or unregistration request.

This ticket will fix it by setting cbparam.expiration field to 0 in case of unregistration and > 0 otherwise.

[ming]

In 4989:

Fixed #1819: Use expiration field to indicate registration/unregistration in registration callback