Make ipfw_nat module use IP_FW3 codes.

Kernel changes:
* Split kernel/userland nat structures eliminating IPFW_INTERNAL hack.
* Add IP_FW_NAT44_* codes resemblin old ones.
* Assume that instances can be named (no kernel support currently).
* Use both UH+WLOCK locks for all configuration changes.
* Provide full ABI support for old sockopts.

Userland changes:
* Use IP_FW_NAT44_* codes for nat operations.
* Remove undocumented ability to show ranges of nat "log" entries.