mod_include: Add an "if" directive syntax to test whether an URL

is accessible, and if so, conditionally display content. This
allows a webmaster to hide a link to a private page when the user
has no access to that page.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@571872 13f79535-47bb-0310-9956-ffa450edef68