Fix the 'nat' option to actually do RFC3581 as expected and extend the...

Fix the 'nat' option to actually do RFC3581 as expected and extend the configurable values for finer control.

(closes issue #8855)
Reported by: mikma
Tested by: klaus3000, file


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@203735 65c4cc65-6c06-0410-ace0-fbb531ad65f3