"globalport" option for multiple NAT instances.
If ipfw rule contains "global" keyword instead of nat_number, then
for each outgoing packet ipfw_nat looks up translation state in all
configured nat instances. If an entry is found, packet aliased
according to that entry, otherwise packet is passed unchanged.
User can specify "skip_global" option in NAT configuration to exclude
an instance from the lookup in global mode.
PR: kern/157867
Submitted by: Alexander V. Chernikov (previous version)
Tested by: Eugene Grosbein
Usual moving of code with no changes from ipfw2.c to the
newly created files, and addition of prototypes to ipfw2.h
I have added forward declarations for ipfw_insn_* in ipfw2.h
to avoid a global dependency on ip_fw.h