ap_expr related fixes/enhancements:

- implement regex backreferences and make them available for setting 
  envvars in SetEnvIfExpr
- implement nested function calls in %-syntax: %{func1:%{func2:arg}} 
- actually implement evaluation of concatenation operator (oops...)
- Fix <If ... > treating an internal error as success


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1037504 13f79535-47bb-0310-9956-ffa450edef68