Add a App-Mode with defined constants

- Normal Mode = 0
- Install Mode = 1
- Maintenance Mode = 2

Moved Install-Mode check to class `App`

Replaced all maintenance and install checks with $a->mode checks

include/dba.php

@@ -93,6 +93,7 @@ class dba {
 
 		// No suitable SQL driver was found.
 		if (!self::$connected) {
+			self::$driver = null;
 			self::$db = null;
 		}
 		$a->save_timestamp($stamp1, "network");