Add some features to ap_expr for use by mod_include:

 * a restricted mode that does not allow to bypass request access restrictions
 * new variables DOCUMENT_URI (alias for REQUEST_URI), LAST_MODIFIED
 * -A as an alias for -U
 * an additional data entry in ap_expr_eval_ctx_t for use by the consumer
 * an extensible ap_expr_exec_ctx() API that allows to use that data entry


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1128564 13f79535-47bb-0310-9956-ffa450edef68