overhaul of ap_ssi_parse_string

- improve efficiency and memory footprint (don't use the power-of-two
  allocator)
- allow the combination of out = NULL && length = 0, which means that
  the resulting expansion is unlimited in length


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101070 13f79535-47bb-0310-9956-ffa450edef68