various code cleanup changes including changing #define'd constants to enums,

comments to doxygen style, memory allocation to use ast_ wrappers, use calloc
instead of malloc+memset, and removing duplicated error messages (issue #6435)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@9259 65c4cc65-6c06-0410-ace0-fbb531ad65f3