Use ast_str_append() instead of ast_build_string() to construct

Use ast_str_append() instead of ast_build_string() to construct
the sdp messages. Overall the code is slightly more readable
(because the string is fully described by a single pointer),
and more efficient (because the length is stored explicitly
so you don't need to do strlen()).
(I have been using this code for almost a year now.)

I wish we had infix string operators to do this sort of things!

Nothing to backport from this change.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@76229 65c4cc65-6c06-0410-ace0-fbb531ad65f3