Add support for IncludeOptional directive added in httpd 2.3.6.

This seems to be used by default on newer systems (at least Fedora 20
has it) causing the apache plugin to not discover large parts of
httpd inventory subtree.

I'm pushing this as a PR to discuss whether we even want this in. IMHO, it is fairly low-risk small change but we've had our issues with apache plugin... ;)