simplify the ast_dynamic_str_*.... routines by

simplify the ast_dynamic_str_*.... routines by
renaming them to ast_str ... and putting the
struct ast_threadstorage pointer into the struct ast_str.

This makes the code a lot more readable.

At this point we can use these routines also to
replace ast_build_string().

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48510 65c4cc65-6c06-0410-ace0-fbb531ad65f3