ipfw: add additional check for orphaned states

[ae]

Summary

When parent rule of dynamic state is deleted and net.inet.ip.fw.dyn_keep_states is enabled, such states are called ORPHANED.
Orphaned states still keep pointer to original parent rule. In case when rule action is skipto this can lead to unpredictable consequences.
To avoid this problem add special handling for skipto action when we have found ORPHANED state.
Check that new rule has the same opcode and skipto number for O_SKIPTO rule action.