mod_autoindex: support XHTML as equivalent to HTML in

IndexOptions, ScanHTMLTitles, ReadmeName, HeaderName
PR 48416 [Dmitry Bakshaev <dab18 izhnet.ru>, Nick Kew]



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@893035 13f79535-47bb-0310-9956-ffa450edef68