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.