Fix zone removal to respect forward configuration inheritance.
    
    Ad-hoc fwd_delete_table() calls did not respect inheritance hierarchy
    in forwarding configuration. Now all manipulation with forward table
    is done in fwd_configure_zone() and fully respects configuration inheritance.
    
    There is a trick: When removing or deactivating a zone, fwd_configure_zone()
    is called with empty configuration set to simulate that the zone does
    not have any explicit configuration. This triggers the inheritance
    logic when necessary (i.e. for the root zone).
    
    https://fedorahosted.org/bind-dyndb-ldap/ticket/167