use ast_build_string() to build strings into buffers, and general cleanup of...

use ast_build_string() to build strings into buffers, and general cleanup of variable serializing functions used by 'show channel' (bug #4558)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5944 65c4cc65-6c06-0410-ace0-fbb531ad65f3