- move the string join() function to utils.c since it is used in both cli.c and res_agi.c

- reimplement ast_join to be of linear effieciency instead of quadratic
- remove some useless checks for "if (e)"
- reorder checks for strings starting with '_' to avoid a useless call to ast_join()
- check array bounds when parsing arguments to AGI
(issue #5868)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@7556 65c4cc65-6c06-0410-ace0-fbb531ad65f3