Limit recursion in ap_expr evaluation to avoid unbounded stack usage

* evaluate chains of ||, &&, and string concatenation non-recursively
* limit other types of recursion to 20 levels
* avoid some string copies if concatenating more than 2 strings


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1204087 13f79535-47bb-0310-9956-ffa450edef68