Add ipfw_get_action() function to get the pointer to action opcode.

ACTION_PTR() returns pointer to the start of rule action section,
but rule can keep several rule modifiers like O_LOG, O_TAG and O_ALTQ,
and only then real action opcode is stored.

ipfw_get_action() function inspects the rule action section, skips
all modifiers and returns action opcode.

Use this function in ipfw_reset_eaction() and flush_nat_ptrs().