Fix most leftover non-opaque ast_str uses.

Fix most leftover non-opaque ast_str uses.

Instead of calling str->str, one should use ast_str_buffer(str). Same
goes for str->used as ast_str_strlen(str) and str->len as
ast_str_size(str).

Review: https://reviewboard.asterisk.org/r/2198
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Merged revisions 376469 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 376470 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@376471 65c4cc65-6c06-0410-ace0-fbb531ad65f3