mod_substitute: follow up r1684900.

Introduce the SubstituteInheritBefore directive to configure the merge order.
This allows to preserve 2.4 and earlier behaviour.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1687539 13f79535-47bb-0310-9956-ffa450edef68