Restore "nat global" support.

Now zero value of arg1 used to specify "tablearg", use the old "tablearg"
value for "nat global". Introduce new macro IP_FW_NAT44_GLOBAL to replace
hardcoded magic number to specify "nat global". Also replace 65535 magic
number with corresponding macro. Fix typo in comments.

PR:		[[ https://bugs.freebsd.org/211256 | 211256 ]]