TS-291, transparent forward proxy, first patch merged to trunk

[Alan M. Carroll]

I did a rebase and merge from the ts-291 branch, which had the changes to 
implement forward transparent proxy (see TS-291). It has had some testing, but 
not extensive. For this reason it was not included in the 2.1.2 release. 
However, because that has now been released it seemed a good time to push this 
non-trivial change to trunk.