Shift XML handling code from Apache down into APRUTIL

- teach APRUTIL how to find Expat and respond to --with-expat
- Apache's configure points APRUTIL's configure at its srclib/expat-lite
  (this will go away; aprutil can work against installed expats or fallback
   to an expat bundled within aprutil)
- shift some of the timing of INCLUDES and top_builddir processing in the
  APRUTIL configure.in script
- expose the new apr_xml functions in apr_xml.h, apr_xml.c (by building it),
  and apu_compat.h
- rewrite util_xml.[ch] in terms of apr_xml


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@88299 13f79535-47bb-0310-9956-ffa450edef68