Open files with O_DIRECT dependent on a compile-time DIRECT_IO #define.

O_DIRECT causes problems on (at least) a wheezy VM, and there are mixed
reports about its performance impact.  This patch makes it a
compile-time choice which should remain until it's been benchmarked.

src/client.c

@@ -485,7 +485,7 @@ void* client_serve(void* client_uncast)
 	
 	error_set_handler((cleanup_handler*) client_cleanup, client);
 	
-	debug("client: mmap");
+	info("client: mmaping file");
 	FATAL_IF_NEGATIVE(
 		open_and_mmap(
 			client->serve->filename,