improve expansion performance.

If we have only small expansions (like just one variable - often used
in map keys or the like), don't stress the pool with allocating
memory for the linked result pointer list. This list can be safely
stored on the stack.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@100901 13f79535-47bb-0310-9956-ffa450edef68