- add XHTML doctypes to httpd.h

- fix the (x)html output of mod_autoindex to be consistent
  and make it configurable to emit either HTML or XHTML

PR: 23747


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101683 13f79535-47bb-0310-9956-ffa450edef68