Allow modules to specify the first module for a sub-request. This allows

Allow modules to specify the first module for a sub-request.  This allows
modules to not have to muck with the output_filter after it creates the
sub-request.  Without this change, modules that create a sub-request have
to manually edit the output_filters, and therefore skip the sub-request
output_filter.  If they skip the sub-request output_filter, then we end
up sending multiple EOS buckets to the core_output_filter.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87065 13f79535-47bb-0310-9956-ffa450edef68