Add extended directory index options: "[no] index" and "[no] auto index".

The option "directory auto index" implements basic directory listing
and is turned off by default.
ok deraadt@

src/etc/examples/httpd.conf

@@ -1,4 +1,4 @@
-# $OpenBSD: httpd.conf,v 1.2 2014/07/26 10:27:19 reyk Exp $
+# $OpenBSD: httpd.conf,v 1.3 2014/07/29 16:17:28 reyk Exp $
 
 # Macros
 ext_addr="egress"
@@ -20,6 +20,7 @@ server "www.example.com" {
 # Another server on a different internal IPv4 address
 server "intranet.example.com" {
 	listen on 10.0.0.1 port 80
+	directory { auto index, index "default.htm" }
 	root "/htdocs/internet.example.com"
 }