simplify the flow of builtin_atxfer().

There is still a lot of similarity with builtin_blindtransfer()
which should be removed by definining functions for the common
pieces of code (eg in the first part).



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@21097 65c4cc65-6c06-0410-ace0-fbb531ad65f3