astobj2: Only define ao2_bt() once.

* Make ao2_bt() not use single char variable names.

* Fix ao2_bt() formatting.
........

Merged revisions 398498 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@398499 65c4cc65-6c06-0410-ace0-fbb531ad65f3