Exporting does not handle associated ALGs and rule procedures

[rmind]

Configuration exporting (npfctl save) does not capture the following:

• ALG and its state associated with the NAT entry (see npf_nat_export()).
• Rule procedure calls and parameters (see npf_rprocset_export()).

The former needs some refactoring: NAT entry should probably use ALG IDs and the ALG API should be extended to support exporting/importing of the custom ALG data. The rule procedure API needs to be similarly extened.