In app_macro, changed the previously changed upper recursion depth limit to a...

In app_macro, changed the previously changed upper recursion depth limit to a variable, default of the original val of 7. MACRO_RECURSION is a channel variable that will override the limit, but until I can understand and fix why this limit is neccessary, I am not advertising this variable in the docs.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@40200 65c4cc65-6c06-0410-ace0-fbb531ad65f3