Merge branch 'apache-httpd-2.4-support'.

This adds support for Apache HTTP server version 2.4 through conditionals, where
the changes are:

 * Use "Require" instead of Order/Deny/Allow.
 * Set DefaultRuntimeDir to a directory within stateDir.
 * Create DefaultRuntimeDir within Upstart job.
 * Don't add NameVirtualHost directives.
 * Use mod_authn_core instead of mod_authn_alias.
 * Add mod_unixd to support User/Group directives.
 * Load the MPM module specified by multiProcessingModule at runtime.