move out another large block to a large function, and document

some possibly missing parts in the privacy screening code.
Now that it is more streamlined it is easier to see differences
in handling the various cases.

Have not tested the code in depth.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@47189 65c4cc65-6c06-0410-ace0-fbb531ad65f3