Provide a way for EXEC_ON_READ directives to see their context

Provide a way for EXEC_ON_READ directives to see their context
(parent directive).

Usually, this context is not also EXEC_ON_READ so it's not fully processed, so
e.g cmd->path is not fluffed up yet because the closing tag
of the section is not read yet and ap_check_cmd_context()
doesn't work)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1656058 13f79535-47bb-0310-9956-ffa450edef68